Caps are definitely hard, getting them right as both a feature as well as setting them as a customer is still an art. This holds true for 'flat rate' too, where your trading correct sizing vs downtime when either get code wrong or get unexcepted success. I've been involved in all these situations so I definitely emphasize.

We had daily hard caps and budgets alerts, but it's still an area we can do better.

(Disclaimer: Product Manager for Cloud Firestore)

There's nothing guaranteed about a "get out of jail free" card, so factoring that into your decision making is probably a bad road to go down.

I'm not suggesting that. For a lot of casual users I'm not sure that cloud services are necessarily the best approach for an externally-facing service. They may well be better with a VPS that has a hard resources/dollar limit.

Perhaps cloud providers should have some sort of hard circuit breaker option (though it won't help for some things like storage) but it's probably not a priority as not a lot of businesses--their primary customers--would be OK with effectively hitting the power button for their entire cloud account if they exceed some dollar amount that someone or other configured a couple of years ago.