It seems like a pretty reasonable concern to me. We can presume the odds are pretty low, yeah. But the consequences are very high - you would need to redesign and possibly rewrite in another language your entire application. People complained up a storm about Reader dying, but it was a 5min process to export your subscriptions and import into another web reader that had basically the same feature set. Conventional Linux hosting, or even Docker, could be pretty easily re-hosted as-is on any of hundreds of other places.

So what do you use as the backend for now?

I usually go with a Django or RoR monolith, something that I know I can run on a cheap DO droplet or similar, and while there's a fixed monthly cost it's reasonable and I can scale up with ease (albeit manually). If I were explicitly needing the realtime DB aspect I'd probably look to Phoenix with the same hardware approach.