Do you know if the vice versa would apply e.g. Jurassic Fibre overbuilding on Openreach?
Cityfibre has built out in Milton Keynes, which had very good pre-existing Openreach FTTP coverage. They must have a viable business case for this.
Ultimately, the altnets compete on price. They cherry-pick the places which are cheapest to build, and wholesale at a lower rate than Openreach. Because Openreach's pricing is regulated they can't reduce it in response.
If a Cityfibre reseller like Vodafone can undercut BT or other Openreach-based providers by 50p per month, people will take it. From this point of view it doesn't really matter whether they are competing with Openreach FTTP or Openreach FTTC - although in areas where FTTC speeds are poor or non-existent, this does give a stronger incentive for people to take the altnet service.
Altnets who build in rural areas have a different business model, charge more, and have a higher risk from when Openreach comes along.