Exactly which network infrastructure are you talking about? Poles: yes, there are overloaded poles in some places. Ducts? In some places there may be congestion from having two or three operator's cables in the same ducts, but nothing like the 30x extra spine cabling that would be required for a dark fibre network. I can't see anything else "overloaded".
It would (and most customers don't care). But you still haven't said how you would get it built in the first place. Try Australia's National Broadband Network for an example of how *not* to do it.
Openreach with a monopoly copper customer base were quite happy to sit on their hands; as long as customers have no alternative, there's no reason to build.
Any mechanism you can think of to get 100% coverage of a dark fibre network, could have been used to get 100% coverage of a GPON network. The handover to service providers is different, but 99% of people wouldn't notice the difference.
And you will pay for this how, exactly?
And you will pay for *that* as well how, exactly? You could of course just build a completely separate fibre network alongside Openreach's network. Which is essentially what the altnets are doing, piecemeal.
A GPON network is very easy to manage. It's just software configuration for moving customers from one ISP to another - zero touch. A dark fibre network involves sending out an engineer to perform repatching, or else an extremely expensive optical patchbar in the exchange.