"Ofcom said at the time "It did not want to set the wrong incentives by making access to BT's existing network a more attractive commercial proposition than building new networks where this is feasible."
I would hazard a guess that it is not "feasible" in over 60 of the country.
This proposal can only increase the cost and/or reduce competition for the majority of the country as it will never be cost effective for multiple carriers to all build their own physical links in low density areas.
OFCOM should focus on getting the fastest, lowest cost, reliable communications to end users not worry about how the players in the middle arrange their back-hauls or some misplaced view that they should force individual players to spend money on costly infrastructure that in all probability will be underused - particularly as the nice 5G elevated masts will allow a lot of traffic to be moved to the ether rather than the fibre.