Actually there is evidence to the contrary. https://www.nextbigfuture.com/2013/02/cable-companies-make-9...
The truth is fiber bandwidth is cheap.
(random youtube video) https://www.youtube.com/watch?v=aUMjCvMxp-M
You don't dig 50m into the ground, unless you were referring to the length and not the depth. But if you did mean depth, I've done designs where the dig depth was under 4 feet. Usually this involved either special products for purpose (Look up Vertical Deflecting Conduit) or literally just a straight trench along a state highway. (Not gonna comment on the security of that practice...)
But if you meant 50m in distance off the backbone, you're right. because that last 50M to a residence -is- the hard part in a lot of cases. That's why one of the things negotiated in a lot of franchise agreements (which range between a permission to build all the way to a locally sanctioned monopoly) includes is a 'minimum homes passed per mile'. Which basically states that if a route passes at least X homes in Y distance, the Cable co cannot charge a prospective customer for the buildout to their residence.
Doing the 'last mile' is the most expensive part of any network.
One might start to ask if this is part of why these Cable cos are so hyped on the 5G; It makes it possible for 'fringe' customers to still be offered wireless products.
What the Tele/Cable cos did with all of that federal grant money in the early 2000s and with the stimulus packages resulting from the 2008 recession was build up their backbone structure substantially. Many of the projects I did were designed to specs along the lines of 'Go up to the next 48 count interval, then double that.' So if we needed 28 fibers to feed a system, we'd put 96 in the ground in that case.)
Which means that rather than use that stimulus money to build out more of those last miles, in most cases it instead went into revenue stream of fiber leasing. Many of the projects I worked on involved Cable companies leasing that dark fiber they built up to the cell carriers for capacity in the rollout to 4G.
Bonus points because in most cases they weren't even doing any multiplexing on the fiber (something that could be done by adding additional equipment but not having to replace any fiber.)
https://nationaleconomicseditorial.com/2017/11/27/americans-...
The unit cost of bandwidth is obviously very low, that's not where the costs come in for ISPs. To suggest that Verizon is on 97% margins while ignoring all of the other costs is to promote financial illiteracy.
If ISPs were actually doing 97% net margins, they'd be the 10x bigger than Google.
The surpluses are in fact in high tech companies like Google and Facebook - they are the ones with 'massive margins' wherein there's obviously more opportunity for competition.
There's probably an economic case for pricing alteration, but gross misinformation is not going to help.
FYI: VZ's actual income statement [1]
[1] https://www.nasdaq.com/market-activity/stocks/vz/financials
Perhaps they can make a case that they're borrowing resources that they'll have to pay back, or causing degradation in ways that will cost them later. But it doesn't seem reasonable to say "we just don't have the available capacity" when clearly they do.