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.