Um, not sure where you supported anything factually, but I'll bite:
1. Class diversity is no better in BFA than Vanilla: see the addition of not 1, 2, but 3 WHOLE classes to the game and still no new pure dps, nothing different in how s class heals, tanks, or dps. DKs are essentially paladins mashed with warriors and warlocks. Monks are essentially rogue/druid mashed with a really cool skin pack. Demon Hunters are essentially hunter/rogue/dks... if I can sum up the design of a class within the design space of another class, it's probably not very diverse. If the game is essentially the same without it, we probably could have used a more diverse design from the development team for that class.
2. Homogenization is the act of giving greater access to tools originally designed to be role or class specific. Giving both horde and alliance paladin/shaman. Making buffs similar and giving every class one. Giving every class an immunity CD and a CC button. Giving every melee interrupts. Every ranged a gap opener and every melee a gap closer. The original version of the game every class had a designed niche it existed in, those niches were as different and unique as the could be: to the point where only alliance had paladins, only horde had shaman. Magic dispelling was done by priests, and diseases by paladins. Poisons and curses by druids. If you wanted to play druid, you had to pick NE or Tauren. If you rolled as a resto druid you could still use almost all your feral/boomkin abilities and swap between tanking or dps, giving you the feel of being a true hybrid. You weren't locked out of abilities unless you went too far into a talent tree to take other options, which was only a very small part of the class you aren't getting access to, between specs.
This is by definition, less homogenizing.
3. Vanilla was a powerhouse MMO before TBC dropped. They peaked subs in Wotlk, around 12m. In vanilla, they had a peak of 8m subs, the most growth they had during any point in Wow. TBC only saw an additional 3m subs, and wotlk the same 3m, peaking at 12m. Vanilla was every bit a powerhouse as it is now, back in Vanilla. The game is definitely more polished now, but it's definitely not any more of a powerhouse, and certainly not because of expansion content.
The other important number to account for, are those published by Blizzard of having over 100m accounts created. While these numbers are old (subscriber numbers also have not been publicly announced since the end of MoP, when they had dropped to 7.7 million, below the peak of Vanilla, LOL), simple napkin math shows that 100m+ unique accounts subbed and never more than 12m concurrent, is a very low retention rate. Simple math shows Vanilla had the most growth, best retention, and overall most ideal design for the largest concurrent segment of the population. While the concurrent sub count increased by 30% and 18% in the next two expansions, it also fell by as much the following two. By the time the 5th expansion dropped, the sub count was no longer being reported because it was most definitely not a good number.
Again, the only way BFA is a titan or a powerhouse, is when you consider the rest of the game with it... which no one does. Because BFA is the only part of it that matters right now. And it likely has something like ~5 mil subs. It would be interesting to see how many of those are only playing Classic. Would be nice for classic to have its own sub fee, so we could see the actual numbers between the two. Can only guess as to why Blizzard won't do that...
Your objective conclusion does not align with reality.