ISPs can. I wrote the code that lets them offer such a service, and some internet cafés in Europe do something similar, but they're pay as you go, so you just

ISPs can. I wrote the code that lets them offer such a service, and some internet cafés in Europe do something similar, but they're pay as you go, so you just top up with a different speed tier. (I can not guarantee anyone is still doing this to today though.) I imagine ISPs do not do this, because of how they sale their service. They want to make it hard for you to get out of the deal you're in, and hopefully in a carrot kind of way where you feel like you're getting a better than normal deal so you don't want to leave. Giving people the flexibility to change their plan willy-nilly is goes against how most ISPs sell their service. They want to lock you in, not give you free range. The exception is internet cafés and hotel internet. I could see something like this for cell phone top up plans, but most of those just resell what's available, which means them giving all of the bandwidth they can get.