Huge advantage for Microsoft to get tons of games, true. That is xCloud's big advantage, it just plays the same Xbox One games, so just needs to have a bunch of Xbox servers (and deals with the game developers). But those games will just be Xbox One S games, while Stadia is a next-gen system, far more powerful than Xbox One X.
Stadia is not that much more powerful than an X. In PC terms it's barely mid-range.
Vega 56 is virtually a potato for 2019, let alone going forward.