Prepaid only is a fantastic idea, until your site goes (desirably) viral and then gets shut off right as traffic is picking up, or you grow steadily and forget to increase your deposit amount and suddenly production is down. Billing alerts are a much better solution IMHO.

No you big dummy, that is especially when you want to limit your liability, lol.

Because these days it will be all worthless bot traffic.

Prepaid/paid limits with shutoff is appropriate for this though.

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.

Let me choose. This common point seems more like a rationalization for the default behavior of hyperscalers. AWS isn't avoiding prepaid due to concern about my site's virality, just that prepaid = less money.

You can also have both, a cap and one or more billing alert levels below it. Some providers do this (e.g. IIRC Backblaze B2).