Communications infrastructure naturally tends towards a monopoly - it makes very little sense for thirty different companies to run wires to your house, on the off chance that you'll choose them over a competitor.

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.