I can't say what's driving all the playerbase losses but: They stopped development on Diablo 3 a long time ago. It doesn't have the staying power of more modern online games where new content is added continuously. They stopped development on HotS, which never achieved the same level of pull as League of Legends. Overwatch faces stiff competition from Valorant and other competitive small team FPSes, and also has over…
In terms of WoW, I'm sure a lot of it is focusing on the top raiding guilds and leaving the casual player with few rewarding activities. Further, one of their most cherished metrics is how much time people spend doing a certain activity. Instead of making those activities more fun and rewarding, they made them take longer to complete and with fewer rewards. To top it all off, they are nearly completely deaf to their…
Not to mention they seem to hate flying yet keep creating terrible zones unless you can fly. There aren't clear routes between common locations like roads in Skyrim. That can be ok, but in that case they really need to have a flatter level design like in Oblivion.