That isn't really true. Let's say we get structural separation and retail competition (which is a fantasy in our current situation). The backbone providers could still become rent-seekers by putting the arm on whatever service lacks technical alternatives to large amounts of bandwidth (which the retail customers already pay for). The retailers have more negotiating power than consumers do, but there is still a place for regulation to prevent a pricing policy from strangling innovation.
The UK has a structural separation between retail and wholesale, and the major ISPs are still desperate to be allowed to do deals with content providers. Consumer interests are not set up to directly disincentivize ISPs from trying to charge content providers for access to them. Consumers will not police that principle.
What would stop backbone operators from doing that? I'd guess the same thing that's stopping them from doing it now: actual competition. The monopolists are only a problem at the edge.
