If Stadia was literally Netflix but with games instead of films and shows, it'd be far more successful right now.
But the large amount of "older" games that would likely make up most of the content of such a service aren't "flashy and splashy" enough, probably.
The proper thing would have been to wait until all the core features are ready for launch, with at least one big internal AAA launch game from SG&E. Even though that would have meant waiting an extra 1-3 years with zero income coming in to support the service (while their competitors catch up too), the fact of the matter is that you only launch once, and launches are important for new gaming platforms.
Not saying a launch completely defines its platform, Dreamcast had a great launch, etc. And PS3 and 3DS proved you can salvage a tailspinning platform but that takes investment and time, too. But the overall attitude of "launches don't matter, we can fix it in post" from Big Tech is pretty much why every new platform has failed since the original Xbox. And by any modern metric the original Xbox was a "failure," selling barely more than the GameCube and losing MS a fuckton of money. But MS was actually in it for the long-haul, and that's always been the biggest question hanging over Stadia's head: is Google in it for the long-haul, too?