Proton is free, open-source, and strives for compatibility with Windows games. Could have used that.
They still get vendor lock-in either way (because you have to stream from _somewhere_) so making it worse in this fashion doesn't achieve any particular goal that I can see.
Furthermore, controlling the full platform has enormous advantages in terms of scalability, low-latency streaming and control.
The hubris to go ahead with something like this....
I can see at least three goals it achieves:
1. It helped to establish the superiority of Google engineering (in the minds of Google engineers), e.g. "We can do it better because we're Google."
2. It helped somebody (or multiple somebodies) in Google get promoted.
3. It signaled to the market that even as a new entry into the gaming market that Google has a big stick to wield and publishers/developers will have to bend to their will, and /will/ do it.
I've played RDR2 singleplayer and I get a prompt about cloud saves every time I launch the game on a new PC.
If he's playing multiplayer, this should be even easier. Rockstar should have the savedata.