Why would that approach guarantee that your speed is consistently high, and why would a no-cap method be limited only to "ultra-cheap" ISPs?
Giving everyone a 1TB cap doesn't prevent congestion,
True enough, but you've reversed what I said to try to create a straw man.
Yes, applying a cap doesn't in itself prevent congestion, but what I said was the opposite way around. If ISPs sell a service at a price below the wholesale cost (because the market is driven - at least in this country - very much by "cheap, cheap, cheap") then they need to find some way to make a profit. To begin with they applied caps (whilst pretending they weren't doing), but as that's now become politically unacceptable to the mass market what they do instead is to vastly oversell their capacity, at the same time claiming, "We'll never slow you down". Then when your connection does get very slow, they say it's not them but other users and you shouldn't be so selfish.
I choose to pay a realistic price for my bandwidth, from an ISP who is perfectly clear that there ain't no such thing as a free lunch, and accept that they do properly provision their capacity (which is why my link stays fast) but that I can't use more than I've paid for. Doubtless they do still oversell - although not nearly to the same degree as the cheap, cheap, cheap merchants do - because there's no way I'm going to use 1TB in a month (typical usage for our house, with a game-obsessed son is about 250G) and they don't oversell enough to affect my connection speed.
You have to accept that there need to be limits somewhere.