It's significant, but it's often done in cities that don't have this regulation problem (see most high-density cities around the world with sane access to cabling tunnels). Or, if you take it to the extreme the other direction, look at all of the municipal fiber projects that end up working out. If a municipality can pay for it, certainly a company would be able to as well.
Laying fiber requires labor, but many of the service tunnels already exist so the only digging required is the last bit from the street to the customer's house.
There is an entire industry designed to help finance capital-intensive projects. Just because you can't do something with a text editor and a compiler in your basement doesn't mean it can't be done.
>but we are talking about a real natural monopoly
No we aren't. Trying to build up an ISP is 95% lobbying local governments. It's only 'natural' in the sense that it's a government created monopoly.
>much more resentment from their prospective customers
You wouldn't be pulling fiber to peoples' homes if they weren't already signing up (see the google fiber rollout mechanism).
>Given the choice, the customers would vastly prefer a local loop unbundling solution
Not if it means the best they can get is 5 mbps over cables that were laid to homes 20 years ago. If your city is filled with a bunch of NIMBYS, they are just as likely to vote down a bond or whatever it takes to raise money to lay in new fiber.
Local governments aren't completely undemocratic. If the local government is hesitant to let corporations start digging up the streets and yards, it's likely that the residents are similarly hesitant to invite that sort of disruption. How many people are actually eager to have a second gigabit-capable cable buried through their yard? Whether or not the federal government intervenes, there will always be barriers to an upstart ISP. Yes, with all local restrictions usurped, a company as large as Google can certainly deploy new infrastructure to select markets where there's sufficient demand for a more reasonable alternative to the incumbent ISP. But that's still a far cry from a functioning competitive market. Who would fund construction of a third set of cables, or the sixth? Even with local restrictions removed, how much capital does it take to build a competitive ISP from scratch (keeping in mind that Google wasn't quite starting from scratch)?
In the most pedantic sense, there's obviously no proof that a functioning competitive marketplace would develop, but we are talking about a real natural monopoly, and that means something. Building last-mile infrastructure is very expensive. Each new competitor faces much worse prospects for recouping their initial investment, much more resentment from their prospective customers, and the incumbent(s) can always undercut you at the drop of a hat. Given the choice, the customers would vastly prefer a local loop unbundling solution—it doesn't have the high costs of building redundant infrastructure, it doesn't make a mess of the town, and it leads to the same end result of diverse choices for services.