You are 100% right, I'm not disputing their position in the market, theyarethe biggest publisher, they earn a fuckton of cash. I'm arguing about the product quality:
WoW: long debate warning, but in my opinion it's less "good" than it was and age is not the main reason. WoD is a bad expansion, because even super casuals are done with it in 2 or 3 months and casual content is shitty bad (apexis, dungeons, lfr).
Diablo: well, if we ignore that it sucked for months after release, it got some players back, it lacks longevity from a certain point of view. What I find interesting is that Diablo 2 which is a lot older, offered a larger amount of builds and customizations, with much more replayability (due to no respec allowable).
Hearthstone bored me inside a week, too simple gameplay, MtG was a lot better.
SC, HotS: not my type of games, can't comment. Overwatch: it spawned from a canceled project, maybe it'll be fun, still not my kind of game.
TL;DR: I played WoW and Diablo, the latter is stagnant and the former is getting worse and sub numbers tell me I'm not alone with this feeling. Of course my opinion won't change a shit, but MoP > WoD.