What irrational fear made you assume something so vast?
not directed at me but i'll try answer since it appears you're curious. judging from the forums and several discords consensus seems to be that bfa (twinking) is about as dog as it gets in regards to gearing, gameplay, competitiveness, activity and community.
In vanilla, gearing is rewarding enough and you don't have to rely on shitty RNG to get a BiS item, while there's still room to acquire "tough" items such as AGM, LFH, tidal charm etc.
As for Vanilla gameplay is what it is; some prefer it and others don't. I'm not too hot on vanilla 19s but others are -- it's not very bursty and centered around the terrain and jumps in WSG. 29s is somewhat a hybrid of end-game PvP (very bursty and many abilities) and 19s (less bursty and less abilities) so it offers a good middle ground. In 39s and 49s most classes and specs are viable and most classes already have their key abilities and talents at these levels and they're not far away from the more fast-paced end game-pvp. Some classes/specs might be classified as broken but compared to other xpacs it's nothing extreme.
Regarding competitiveness, most vanilla pservers have had somewhat weekly premades in both 19s and 29s (a few premades at 39s also but too few to really deserve a mention) the past 5-6 years, with rulesets to promote competitive gameplay. I don't think the premades are as skillful as some of the earlier twink cups (or the games leading up to them) but that's hopefully gonna become better if said teams and players migrate to classic. There are also a few instant 60 pservers that host premades on the daily, and to my understanding these servers will still be up after classic retail has been launched.
As for activity, classic will have cross-realm BGs when when WSG gets implemented (phase 3) and twinks/non-twinks aren't separated so it's more than likely that 10-59 will have instant pops during most hours.
And lastly, community, 19s, 29s and 39s have been populated by the same core of people for many years and some twinking guilds have been around since Nostalrius times. Idk how to measure whether the community is good or not but the fact that some guilds and ppl have stuck around on the same servers in the same brackets for a very long time speaks (to me) in the direction there's been healthy communities and that's likely gonna continue on classic retail.