So? Lots of studios have flops. The point is they've had plenty more successes since Skyrim. I even forgot to mention DOOM and Prey. And don't forget, FO76 wasn't even made by the main studio.
Adding the back catalogue to gamepass (and probably taking it all off steam), is part of the price too.
Hell, if the price was $7.5 billion even after the FO76 flop, what would it have been if they'd pulled it off? Making a whole new type of MMO.
Fallout 76 is even the exact sort of "live service" game that seems to do really well on Xbox Game Pass. (Sea of Thieves is often mentioned by Microsoft as a gold standard live service game that exceeds exceeds "sales expectations" precisely because of Game Pass; it probably wouldn't have sold as much as it has without Game Pass.) Rumors are that when Bethesda put FO76 on Game Pass the player count went way up, though Microsoft remains mysterious about actual Game Pass numbers, and FO76 also released a major expansion pack and cross-play at the same time, so if those rumors are true it may not just have been Game Pass but the confluence of things.