I don't think that's right at all, and to look at it from an overall view, Legion from 7.1.5 up to and including 8.2 is the most balanced game we ever had when it comes to dps throughput. Of course there have been issues, but that's been the case since the start. In WotLK we still had "hybrid" tax(or was it?) were we had several specs underperforming the whole expansion. Cata had severe issues at start of the expansion. Balance was indeed better in WotLK than TBC/vanilla which were awful when it comes to balance, and Cata even better than WotLK, but I was playing lesser performing specs in WotLK and I know how scaling for some specs were non-existent and got buffed in every patch because they were so behind.
It's not really that black and white, and it's just not as easy as to just point to that and say it's unbalanced, especially since they have proven that they can balance it better with them compared to without. And yes, vice versa.Especially in the light of the fact that Blizzard has gained amazing tools to actually balance things seperately, back in Wotlk, they just couldn't buff spell X because it might make some hybrid build too attractive or buff another spec (which doesn't need a buff) that happened to use the same spell.
Not to mention that PvP was a massive issue as well, as didn't started to balance PvP seperately from PvE, as they can do now.
The "tuning knobs" (and i don't mean any external power systems) nowadays dwarf what the developers previously had to work with.
This though, I 100% agree with. BfA was pretty well balanced all the way until 8.3, and the reasons you put out here are the reason for it. It was a system that shouldn't have been brought in the game with how horrible tuning was. As some specs scaled immensely with one stat or two, while other's didn't get that benefit. Then we had the proc ones which favored some specs, while some not at all.Those corruptions and legendaries involuntarily affected the balance of power between Classes, especially Corruption.
Mages with full Mastery amp. Corruption aren't nearly as OP than without them.
Matter of fact, BfA actually recovered in terms of overall balance since 8.1 and 8.3 was in terms of class changes also a step in the right direction.
Then Corruption came and fucked everything up, because anything that doesn't scale utterly insane with any stats (or can bypass the negative effects of high Corruption as easily) has been pretty boned.