Unlimited* plans are always sold on the idea that a sizeable part of the user base aren’t going to use an actual unlimited amount of the resource.

Unless there is a contract regarding a fee over a period of time, there isn’t that much that users can do to compel a service to offer a service they no longer want to offer.

Absolutely! But I don’t think that’s the point of contention here. The problem is the “abuse” rhetoric, since it’s not just incorrect but disingenuous to basically claim that the users did anything wrong here. They’re imposing limits because they miscalculated how many heavy users they could handle.
Again, that’s a completely reasonable move, but framing it as anything but a miscalculation on their part is just a dick move.