Can get a chromecast ultra on "Kleinanzeigen" (craigs list) sometimes, but many of us have already some or more of these fireTV or sticks. Youtube is running on those, too. So it could be a blink of an eye for stadia to offer an app for fire tv and people can play there with there controllers.
I know, Google/Amazon are not very best friends, but anyway.
@Phettsack I think you actually hit the nail on the head with the problem when you mentioned the relationship between Google and Amazon. Amazon has only recently added back the YouTube app because sometimes they disagree and remove each other's stuff. It's one thing to not be able to watch YouTube videos because Amazon pulled the app. It's a whole different level of problem if people can't access their subscription-based service and play games they paid money for if Amazon was to pull the app.
So I don't think it's likely that it will ever happen. It would be nice for sure, but it's unlikely.
@Phettsack - Other big issue here is hardware constraints from the casting device themselves. Keep in mind Stadia currently only works with Ultra. Will we see that change for other Chromecast devices? Likely (eventually). But with Amazon announcing they'll be getting into Cloud Gaming as well, I doubt we'll see interoperability any time soon.