You're likely concerned about the precedent of Facebook shutting down Parse back in the day, leaving many projects stranded. Facebook though had no real commercial interest in cloud services back then.
Enterprises really like having someone to sue when things go wrong. And indie devs who pick BaaS offerings do so in large part to avoid the hassle of self hosting.
I'm not sure if it's still valid but it's not too hard, and still much easier to maintain than dealing with AWS. I have basic AWS proficiency, but it's still good to have Heroku to not think about it.
One big part of Flutter was that it was a hedge against Java lawsuits back in the day. The other was that it was experimenting with some mobile code, like designing it around lifecycles and all the async UI. But now the same paradigms have been adopted by iOS and Android.
Unlike BE, mobile moves quite quickly too; the last few years have been deprecating a lot of things like push notification security and file access. Mostly to deal with dark patterns. So a platform that isn't in active maintenance tends to fall apart quickly like we've seen with many mobile libraries, Cordova, etc.
Firebase hosting only charges you when an actual thing you've deployed starts getting significant usage. It's a terrific option for small devs who still want custom domains, backend data, and the freedom to code in JS/TS in the browser.
As a small dev firebase is horrifying.