I'd argue that a user generally has an idea that there is a persistent storage ("disk") locally held in their computer and that they have a good idea whether they want a website to use that space or not.
The developer lacks the knowledge of the users requirements, that is why they can't answer the question. For "power users" the user is far better placed than the developer to answer the question about how much local storage space is used.
For a naive user the question comes down to "this website wants to put stuff on your computer, do you think your interaction with the website warrants them doing this", that's more a question of value of the website to the user than it is a technical question.
> and that they have a good idea whether they want a website to use that space or not.
"What's a website? I just double-clicked on my e-mail google and now Foxfire wants to fill up my disks. Is this going to put a virus on my Microsoft? Why don't they put it up in the clouds?"
Most users would just answer yes, thinking bad things might happen if they run out of 'disk space'. So you'd still need some kind of eviction strategy for people that never said no.
The developer lacks the knowledge of the users requirements, that is why they can't answer the question. For "power users" the user is far better placed than the developer to answer the question about how much local storage space is used.
For a naive user the question comes down to "this website wants to put stuff on your computer, do you think your interaction with the website warrants them doing this", that's more a question of value of the website to the user than it is a technical question.