People say that Cata killed WOW. I agree overall, but for me back in the day the biggest single ‘unsub event’ that I witnessed was TBC (original, not classic). It wasn’t the content of the expansion, but the CONCEPT of the expansion. The high-end raiders hated the fact that it would negate their months/years of effort. There were four big raiding guilds on Alliance on my server (Dragonmaw-EU) in Vanilla, one survived into TBC and the other three had to merge - and that was going from 40-man to 25-man. Almost everyone I knew in Vanilla quit, except for about 10. I guess the later generations of raiders got used to the fact that each expansion had a limited lifetime, but people didn’t realise/expect it the first time it happened.
Regardless, I think they should maintain a WOTLK-Era server for those who don’t want Cata.