Historically, most places with government owned telco's starting ISPs still had arms length regulation. If
that was really the concern, looking at the dozens of countries with experience in regulating this would be rather simple.
That said, I agree that you'd get far just by providing the last mile. In the EU, deregulation basically required the owners of monopoly last mile infrastructure to separate them out into separate business units that are heavily regulated to provide equal service. Nothing prevents providers from choosing to build their own, but this ensures "anyone" can start an ISP.
E.g. in the UK, BT separated out OpenReach - not only are they restricted to offering same service and prices to everyone, their wholesale prices are published on their web pages for everyone to see.
There are still problems - e.g. BT is often accused of milking OpenReach rather than investing in the network. But those problems could have been solved by regulating profit-taking so that profits can only be taken as a proportion of investment.
OpenReach offer both local-loop-unbundling (IPSs put equipment in BT exchanges and handle backhaul themselves, and get a raw copper or fibre connection to the subscriber) and backhaul where ISPs connect to BT one or more places and get a raw IP connection to the subscriber.
It works ok, with the caveat above, and it also doesn't stop alternatives, like FTTP from other providers, in areas where it is economically viable to lay new networks.