I think I addressed that earlier. In the range of potential build performance (damage, tankiness, clearspeed, etc.) the outliers are not supposed to be too far from what other builds are lower down on the hierarchy. This is not an exact science and is somewhat intuitive but very much based on the gaps between certain builds at the top as well.
For example you might have two builds that do shard 88. But maybe one does so with many deaths on the way and the other can do it feeling completely safe the whole way. This is frowned upon when this high up is already an outlier. But that doesn’t even mean the safe build needs to be made unable to do it or something. A trade off like speed getting there for that safety can make sense, like Octavius.
The problem here is that it is SO multi-factored man. I think this is why a good way to look at buffs/nerfs is trade-offs. But even that isn’t enough to consider. You have to consider, are the two classes naturally synergetic? If so then they should get a bit more range toward the top to perform well, to reward players for seeing synergy and capitalizing on it. (Good builds should be intuitively so, while it’s also fun and encouraged that some other weird zany genius build designs that are un-intuitive maybe can make it to the top.)
Is the gear near impossible to get (lives and dies on proper MI rolls, high rolls)? Then it’s probably allowed more room to lean toward “OP.”
But back to trade-offs (aka the soldier problem LUL). Generally you have ability to survive, ease in that ability to survive, ability to clear quickly, ease in piloting to clear quickly, ability to do high single target dps, ability to clear crucible, ability to clear SR, speed in clearing campaign (shadow strike spellbreaker and vanquisher vire’s might builds might not be top tier in challenge content but they can farm specific campaign bosses faster than anyone due to mobility), ease of acquiring items, etc.
Being strong in some areas is meant to come at a cost in others. More synergetic builds will get leeway to be good in more areas but it shouldn’t be a king of everything. Ideally squishier stuff would deal more dmg or have more crazy AoE. Tankier stuff would clear more slowly. But again these are generalizations and other factors are weighed in.
As you said, if we ended up in a scenario where pretty much every build was hitting SR75ish and hitting a wall, but all of a sudden a build comes out that is hitting 90. That would certainly be considered a nerf. But even then you’d have to look at how it does it. Does it crawl on the way there? Taking multiple times longer to do it than every other build? Maybe it’s not as egregious then.
So reasons for nerfs aren’t just for SR though. Crucible DOES matter to the devs. I mean they did make it lol. I think the vision is to bring things closer to 7 minute clear times as top tier. (and if SR drops absolutely shit on crucible’s too much then it’ll get tuned or Crucible’s brought up I’d imagine).
Finally there are some nerfs that players don’t tend to like:
Thematic nerfs/nerfs of things that are simply not performing as intended.
Take starpact which was basically picked by every single aether build in existence despite not being the aether exclusive. Some players might find it cool but this is not ideal from a dev standpoint I think. Some might say “Just buff reckless power!” But the Devs might feel reckless power is actually strong enough, it’s just that the other skill is stronger and also offering something that cuts into the territory of reckless power for aether builds (CDR) so a penalty was added (conversion to cold) to make it function for its intended build types. Oh and by the way, that did open some cool and funky aether to cold builds.
So while I broadly stated above stuff that goes into nerfs, there’s also just cases where changes are made so something works the way it is themed to.
Finally, they’re only human and they do care a lot about the community. More than I think they get credit for. If the community is absolutely united in something by a large majority, then even if the devs may personally think otherwise or like otherwise or whatever, it goes under extremely strong consideration and from what I’ve seen they tend to be giving in these circumstances.
And also about being only human, sometimes they try things in balance changes that just didn’t go the way they intended it to, or went too far… it happens.