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…
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 player's concerns, and instead focus on metrics instead of gameplay or fun activities.
People at Blizzard certainly have forgotten that games are supposed to be fun.
They try to cloak their declining userbase by saying they have the highest number of logins ever, but they quit reporting individual subscribers a long time ago. One of them is a completely worthless metric and the other keeps the lights on.
The last of the original WoW dev team quit Blizzard last year, and they are running with a crew that don't have the chops to make engaging games.