How does this actually work? It clearly can't be forever, since any non-zero dollar amount * infinity months is infinity dollars, which is going to reduce the budget below zero since any non-infinite number minus infinity is less than zero... thus locking it immediately.

Even if we say "you get N months of storage before we delete it" and subtract N * current storage cost/month, what happens after you're locked out of all actions because you added an extra GB? Storage APIs cost money to use, so you would get locked out of those too (note that if you're not, people would set arbitrarily low limits and get storage access for free) and couldn't retrieve anything. The only remaining actions are delete (which is free) or raise the quota and do the whole rodeo over again.

Abuse is impossible to ignore at public cloud scale, so "free storage forever" (or even, storage at a one time fixed price) as the fallback isn't a viable option.

Lastly, from an optics perspective, which blog post would you rather see on the front page of HN: "I did something dumb and spent too much money on Cloud" or "Google is holding our data hostage" (or "Google deleted all my data")?

Source: I launched Firebase Storage, which has a GCS bucket that has a hard limit.

For it to work, obviously the budget has to be per month (for instance, $100/month), instead of an absolute limit. Most of the time, that's what you'd want: if you calculated that what you use will cost $50 each month, setting a budget of $100 per month would give some room for growth while preventing billing disasters (and you can always increase it a bit if necessary).

Off the top of my head I'd say that if you're budgeting for storage, the max you can afford for the time period that you'd need to recover data in the event of a budget overrun taking into considerations the delay time for notifications would be a way to calculate that. And that sounds like something that is reasonable to put on the customer to calculate.