Yea but all you can eat buffets have a clear limit: The stomach size of the guests. It’s not an unlimited dinner. It’s specifically limited to the amount you can eat. (Besides that, a lot of all you can eat places have a time limit of an hour or sth).

If dropbox or google offer unlimited storage, then it’s only reasonable to use that storage. After all, that’s what you signed up for. It’s not abuse if they tell me it’s okay beforehand. As long as the terms of service don’t specify a limit, there is none. And if the terms of service do specify a limit, then unlimited is false advertising. If they don’t want you to use as much data as you like, they should have called it the 20TB plan or whatever they see as reasonable.

A way to offer unlimited storage but "cripple" it enough, so users won’t fill your server quicker than you‘d like, would be to only allow a certain size of uploads per month. So you have unlimited storage but you can only upload, say, a 100GB a month. That way, it‘d take almost a year to fill up a Terabyte and you can still claim unlimited storage. That would of course also cause backlash but you could technically still offer unlimited storage.