Exactly, in fact, Stadia is really just a way for Google to subsidize their cloud GPU buildout. Do you think those expensive GPUs are sitting idle in the meantime? No way! They're crunching numbers or spot instances for AI/ML workloads.
The OpenCL ML ecosystem is really lacking at the moment.
(Disclaimer -- I'm one of the devs. Even though we were bought by Intel a year or so ago, we've been pretty good about keeping the AMD / OpenCL support intact.)
The better "guarantee" on that end is the knowledge that Steam makes Valve lots and lots of money so they'll probably want to keep it running. The fact that you do have some sort of local copy and cracking would be an option if it came to it is some comfort also, though that's not really a solution that scales to the size of peoples' massive Steam libraries. There's also of course DRM-free stores like GOG as an alternative.
1. If Steam shuts down, Valve shuts down. If Stadia shuts down... Google is doing just fine.
2. What Stadia is attempting to do is much more technically challenging, and I've yet to see a strong reason people would want it. Steam fills a clear and unambiguous need, and does not depend on near-magic technology to work.
3. Steam works, right now, and is reliable. Stadia? It's unlikely to ever be reliable for many users. Maybe it will be, but that remains to be seen.
It's conveniently indexed by year.
The model where you're still "buying" the games ties you to the ongoing operation of the service much more strongly than in many of their services where an eventual migration might be a little painful, but it's still possible.
The counterargument would be that they've deployed a ton of network capacity and GPU resources for cloud computing anyway, and so Stadia is just riding on those coattails... But it's certainly not a promising track record.