So it took me a bit but I've finally been able to create a proper model for stagger for build 21414. I'm just going to get straight into it. There's not really going to be any pretty pictures this time around but if you're interested in the raw data you can find it here, on my theorycrafting spreadsheet. For the sake of clarity, the model starts at 0 seconds in a fight and models for attacks every 1.5 seconds, which are assumed to hit. It then compares how much damage you would take in the next 6 seconds if you either purified and dropped ISB, or maintained ISB at that point in time. A green highlight means that purifying is guaranteed to be better from that point forward and an aqua highlight means that purifying is better at that point but it dips down at a later point.
The primary goal of this comparison was largely to determine what significance, if any, these changes have on our core gameplay: managing purifying and maintaining Ironskin Brew uptime. The findings were pretty conclusive: while there is a difference and it takes longer for purifying to become the right choice if you need to give up your ISB buff, it is only a matter of about 1.5-3 or so seconds, or the equivalent of one or two boss attacks using this model. So as far as the rough math goes, purifying is still useful and our core gameplay remains more or less intact. In reality it will take longer because you're going to be doding attacks more often but ultimately there is a point where you do benefit more from purifying.
The biggest difference is that purifying now reduces about half as much damage as before as far as how much I've modeled is concerned... which isn't unexpected. There's some worrying consequences though for intuitive use of the mechanic. Around every point on the table where purifying is better, there's about a space of two or so seconds where the numbers are virtually indistinguishable between using PB and using ISB. Personally speaking, I was taught that if a game required external assistance for a player to appropriately learn a mechanic, then the design is not doing its job properly. While this phenomenon occurs for PB when it purifies 100% of the stagger pool, its much less dramatic. The time it takes to see a significant difference in the damage taken when purifying in comparison to maintaining ISB is much lower than when PB reduces the stagger pool by 50%. This has further consequences in that players will take longer to feel out when it's better to purify, which means they will take more damage as they hold off on purifying, making the nerf even greater in magnitude than just a 50% reduction.
There is actually another worry here that in the long-term of a fight, it's an even greater nerf. Because you can't totally remove the stagger pool via gameplay, over time it will continue to increase. This means that while you can purify at any point, by the end of a fight, you WILL be taking more damage per second than at the start. This amount could possibly be not a whole lot, or it could be more depending on how much purifying is practically necessary. This has a lot of consequences for gameplay and design. If you are constantly tanking (or at the very least tanking so often that you do not have 10 seconds where you don't take damage), you are ultimately liable to just take more damage unless you are fairly liberal with your purifying and even then it just delays and suppresses this problem, rather than fixing it entirely.
I see this having a notable chance of messing with balance in the new Mythic+ dungeons which are supposed to more or less be the new Challenge Modes. You're on the clock and if you spend 10 seconds waiting for stagger to tick off every couple of packs, that's a lot of extra time that you've wasted. Or if you only wait before bosses, you're potentially liable to be chunked on trash and that's not good either. On top of that, there's some affixes that could be very bad for Brewmasters, like the constant damage of Decay. You're just piling up constant damage and even if Decay isn't affected by Stagger, that's just even more of a constant mana drain for healers. In combination I think it's fair to worry about the viability of Brewmasters in this environment when you're talking about the higher difficulties.
This is also a potential problem for hardcore progression. While I'm not going to say that the game should be balanced around a very small minority of players (sorry guys), the worry that a Brewmaster would be taking more damage at the end of a fight than at the start, compounded with the ends of fights largely being tougher on everyone really makes Brewmasters seem like a bad class to bring. That is, I think, a very strong worry.
The change to purifying I think reinforces a fairly valid concern about Brewmasters and that is that we actually have very little control over how much damage we take. Our emergency heal is pseudo-randomly procced and requires us to move around which is potentially not a valid option depending on the encounter or the point in time in an encounter. Our cooldowns are very long and realistically not useful in moment-to-moment situations if you are on an encounter with sufficiently deadly mechanics that you need to save these cooldowns for. We have no passive mitigation so damage taken is roughly equivalent to a DPS without purifying, and now purifying can't even fully get rid of the amount of stagger we have which according to feedback that I've seen does not feel good. I realize that this is in part necessary because of the paradigm change from Warlords to Legion. Tanks as a whole are supposed to rely more on healers. That being said, I thing being able to purify some damage is important for that as well. We're giant mana sponges without it. I'm just a humble theorycrafter and budding designer. I'm not a Warcraft dev, I don't have the specific experience of being a class designer on World of Warcraft. Perhaps I'm just over-sensitive to the potential of this change. That being said, I still do think that changes to what we have wouldn't be a bad idea. There are other avenues that could be explored to balance Brewmasters.
Ultimately if the big reason Purifying Brew was nerfed is because Brewmaster's capacity to reduce the amount of damage taken is too high with it (which I don't know if this is the reason, I'd honestly really like a clarification on this still), then you have to ask how to try and rectify that while bringing purifying back up to 100% of the stagger pool.
You could reduce the amount of stagger Brewmasters have, but that has a lot of worry about taking way too much burst damage for the tools we have to mitigate it and that's already a concern of players (which admittedly isn't a foolproof metric for balance). You could reduce their self healing, but there's frankly not that much there in the first place. Reducing health doesn't reduce the amount of damage you mitigate and would make it easier for Brewmasters to be burst down. You can't really reduce the amount of passive mitigation they have seeing as they have zero from being a Brewmaster.
I think that strategic buffs and nerfs would be appropriate. Perhaps have less stagger from ISB but at the same time, give some passive defense and/or some more self healing. Just doing something like this would be much easier to test across various encounters and content to see whether or not it's good balancing and I think ultimately it would be the best for everyone. I don't see messing with purifying brew as an easy method of balancing the spec. There's just so much going on under the hood and it's exceptionally difficult to realistically model the consequences of that without scripting.
To wrap it up, I think the changes to Purifying Brew are... complicated. I think there's a lot of potential "down the road" consequences for it, especially in various content where downtime is not really available. Being said, the model I made really didn't have that significant of a difference for at least gameplay, so that at least is largely okay in the sense of actually needing to purify at all. Still, the model's worrisome because there's a larger period of time where as a player it's harder to tell when purifying is useful, so there's a double-dipping of increased damage taken. Overall I would like to see some changes and I can only hope that Blizzard appreciates this post if anyone from the class team comes across it. I'd like to say that I've been fairly reasonable and objective in this feedback rather than alarmist, but ultimately that's for Blizzard to judge.
Thanks for reading guys. Sadly, I don't have alpha myself so if people would be interested in trying out the stuff I mentioned here and seeing if it's worrisome, that would be appreciated. Any more specific data we can throw Blizzard's way the better.
~The Brewing Scribe