The advantage of the Openreach model is that the last-mile infrastructure is treated like a public good. Anyone can lease bits of that infrastructure and everyone pays the same regulated rates. The system ensures that the monopoly is as small as possible, segregating out the stuff that's a natural monopoly from the stuff that naturally supports competition.
Thanks to Openreach, I have a meaningful choice over my internet service. I can choose dirt cheap service from a multinational, or I can pay a premium for specialist service. If my ISP starts doing traffic shaping or has lousy customer support, I can choose from any number of other ISPs.
Agreed, but suggesting "America is supposed to be about free markets" therefore it should have a monopoly isn't a compelling argument.
It should be a monopoly despite the normal free market model, not because of it.
To be fair, you've just replaced a oligopoly with a monopoly.
Openreach is a government-controlled monopoly, but US telecom is heavily regulated too. Both charge what the government lets them.
As you point out, rural support is a lot less if no one can recoupe infrastructure expenses. The UK has 5x the population density of the US.
So...you might be right, but it's not a clear-cut answer from basic principles.