In 2008 Blizzard released Wrath of the Lich King. The sheer size and scope of the zones along with the intricate attention to detail, in conjunction with improved class functionality and increased focus on specific areas of the game (i.e. heroics, raiding, arena) set the bar for the entire MMO industry.
Fast forward 12 years, and you have the Shadowlands expansion. Which basically consists of the same square footage as six football fields, with the worst game design in all the aforementioned areas of any MMO in the last decade. But at least you have Pelagos.
Honestly, when you look at Wrath then look at Shadowlands, you can see a clear difference to why one was successful and the other failed.
Wrath Classic is successful, mostly because it was a game for everyone. It provided everything that benefited the casuals while also providing the challenging content that the hardcore players want, you can easily get gold wotlk.
Fast forward to Shadowlands, and while the same mechanics to wrath still holds true, it actually failed because of what came afterward. The borrowed power system and the endless grinds that came with the end game, it really destroyed the game. Plus, with the level scaling that was introduced in patch 7.3.5, and the level squish introduced with Shadowlands, it sort of really did kill the old world in worse ways than anything.
Plus, the in-game cash shop and micro-transactions, don’t make sense for a game that has a subscription required to play.