This is fallacious, as capacity is consumed and is a limited capacity.
So, if I understand you correctly, if I download 1GB, then for the rest of that month, the routers can only handle their normal traffic minus 1GB? And so on for every user?
No, your explanation is flawed because it assumes that, at a given time, one individual can consume all the bandwidth, locking out other users. In reality, of course, what happens is that everyone's Internet service slows down so that the sum total matches the capacity. But the network capacity for the rest of the month is unaffected. Even if no one downloads anything, the network capacity for the rest of the month is unchanged
Network capacity has an instantaneous limit, and capacity is not "consumed" in any meaningful use of the word.
What ISPs are really selling is speed. But this ISP only offers low speeds. It's offering a sub-par product and would like to hide that in ridiculous monthly caps.