I suppose it remains to be seen how successful Stadia will be at pull these titles to its platform. I think you're right to worry about fragmentation. If developers view Stadia as "just another platform" that they can just choose not to support when creating exclusives, it'll fail. If Stadia can get them to view it as a kind of drop-in that lets a much larger number of people (with, say, underpowered hardware) play their game, they'll be more likely to view it as a win.
One other plausible market: people for whom the upfront cost of a platform is still too high. It's a lot easier for most parents to justify buying Cyberpunk for Stadia for their kid for Christmas than it is a brand new $500 console, or God forbid the several thousand dollar PC you'd need to play it.