Cheaper montly plans if we don't upgrade?

Who will be the first one to do this?

If I don't upgrade my phone, they should lower my monthly fee, right?

So far everyone is focused on paying more to get newer phones more often, but what about the alternative? No upgrades but lower monthly payments.

Why hasn't Verizon/AT&T/Tmobile/Sprint done anything like this yet? They would definitely have my $$$.