Interesting thought. I then wondered if perhaps we could introduce some kind of mechanism like a token that is authorized to spend a certain amount, and then wondered if we could take that concept to the extreme and have one key with some money loaded that users could put into websites. it was at this point I realised I'd just invented the credit card.
Such a service already exists - https://openrouter.ai. They're basically an aggregator of a lot of LLM models and APIs with unified billing (with both Stripe and crypto). And the main part related to your comment is that on OpenRouter you can create capped API keys with a specified limit in credits.