Or if the government actually used its regulatory power and mandated that companies need to lease last-mile to anybody who wants it at competitive rates.

Competitive rates means not at a loss to the provider - so the libertarian can please stop screaming. At the same time, last mile should be treated as a commons, simply because having everybody provide that doesn't make a lick of economical sense.

(And if it has to be, split ISPs and last-mile providers. You can't run both. That removes a giant conflict of interest)