What killed raiding is the revelation that M+ is todays standard of accessible while raiding itself is still stuck in 2005. Who wants to maintain a 20m roster just to try mythic? Excessive time wasting with too much trash (some is ok and good), extremely long run times when dying or having to summon someone because you miss a Warlock. Having to eat food every wipe. Unforgiving and unfun mechanics. Having to carry dead weight cause of the 20m structure which results in pure frustration and people leaving -> guild dies. There is so much work that has to go into a 20m mythic guild that most people just don't wanna bother anymore. All the log analysis, maintaining a roster, recruiting, trying to figure out how johnny can not die the 100th time and wipes the raid.
In todays age its a failed system. Some stick to it because the only alternative is M+ which isn't most peoples cake. What WoW needs is the return of 10m mythic, better boss design in terms of fun mechanics and not punishing the whole raid for the mistake of 1, cross-realm from day 1, better lockout system and less time wasting as in running back to the boss after each wipe etc.