Look at when Blizzard tried starting new a new IP with Overwatch, I'm sure they did okay but Overwatch doesn't have the same legendary luster that Warcraft, Starcraft and Diablo have (or had).
Overwatch is definitely a big success - it's not the biggest competitive FPS on the market (probably fortnite if you count that - otherwise maybe CS:GO?) but it's up there. They also own Hearthstone which baffled me on release since it's so far out of their wheelhouse - but I believe they're making bucket loads of money off of that still... it's a literal collectable card game ><.
From a raw sales standpoint Overwatch has done better than most of those games. The only Blizzard game that beats Overwatch in terms of raw revenue is WoW. I find that tech people that used to game in the 90s tend to way overestimate how popular and successful those early foundational games were in comparison to modern games. Overall growth in the game industry (especially PC world wide) has been massive
