The way I've always prepared for that was to run things on fixed-cost hosting and if it crashes, it crashes. No way I could get a 10k€ AWS bill overnight if my project blew up on HN because they all run on one dedicated machine that costs me almost exactly 1 espresso per day.
If people end up liking something enough to need more capacity, I can scale the server manually to however much I'm willing to spend and immediately set up Patreon/Kofi/whatever. If people contribute enough to pay for a bigger server, I do that. If they don't, it's their problem that it's slow or keeps crashing. My IPs are always prioritised by the load balancer, so it makes little difference to me.
I had a similar idea a few months ago, basically have a patreon/whatever monthly goal, if it doesn't get hit (enough to pay for the server costs), it doesnt get paid for and goes down. You could even have a "stretch goal" for dev time (basically what would be your profits), if it earns enough you spend time fixing bugs and adding features, if it just gets enough to keep it running, it just keeps running.
It seems like a nice and safe idea, but it does somewhat limit profitability I suppose, but thats an entirely different matter
If people end up liking something enough to need more capacity, I can scale the server manually to however much I'm willing to spend and immediately set up Patreon/Kofi/whatever. If people contribute enough to pay for a bigger server, I do that. If they don't, it's their problem that it's slow or keeps crashing. My IPs are always prioritised by the load balancer, so it makes little difference to me.