If there are no regulations here's how it goes: first ISPs compete in urban areas, this is very profitable with traditional active fiber to the building and ethernet to each apartment. It takes like $50 CAPEX per customer. But at some point as competition increases margins get lower and certain ISPs start looking into rural areas where they can have no real competition, i.e. DSL or radio is not a competition to fiber. New ISPs emerge too as people see an opportunity to profit from temporary geographical monopoly. So they start building in rural areas, enjoying huge margins. Today they'd use something like GPON, which is very cheap compared to the old days. It goes on until at some point the hard part becomes finding areas from where they can profit the most and where there is no real competition. This is when some start competing with other ISPs in rural areas. But as margins start to shrink in some areas due to competition they prioritize investment into areas competition unlikely to reach. There will be far away neglected areas still, but here's a twist, since there is a lot of competition people who are willing to pay will be able to find a whole bunch of ISPs willing to build fiber or at least radio links with them. In other words - it's always profitable, regulations really is the only problem.
Forcing to share infrastructure is also not necessary. It helps only if monopolies share it with everyone, not small players. With no regulations market solves this pretty well.