Prepaid only is a fantastic idea, especially for dumb-ass startups. Limiting your liability to $100 or so sound like a big-ass W.
No need to retire API keys.
It's pretty easy to get right, if the provider allows you to go (slightly) negative before cutting you off.
> Also, can you imagine the kind of downtimes and complaints that would inevitably originate from a fully synchronous billing architecture?
Doesn't need to fully synchronous.
If you have per key limits, this is not possible, and even in a wild situation you should b able to expect that your firebase key will not use 50k.
Because these days it will be all worthless bot traffic.