> there are no plans at all to improve the last mile infrastructure
The economics of laying fiber to the premises are heavily driven by density of potential customers * probability that a potential customer will sign up if you run fiber down their street. You can get reasonable intuition by focusing on the density alone & ignoring the competition / pricing side of things.
I'm not familiar with current US construction costs to install fiber but have some intuition from Australia.
With Australia's national broadband network project from a decade or so ago, it'd cost in the ballpark of $100 AUD per meter to run fiber down the street in an underground trench - most of the capex is digging the holes in the ground etc, the cost of the cable itself is essentially free. The construction budget for a suburb would be something like $2000 / premises. To give a ballpark estimate, suppose 25% of your budget is the premises-specific work to connect the house to the cable running down the street, if they choose to sign up for your broadband plan. That leaves at most $1500 / premises for the rest of your capex budget, so the economics only work if you've got a neighbourhood with a density of least one house every 15 meters. Those numbers aren't exact but they'd be in the ballpark.
There's a bit of variability in the cost per meter to install, if you can reuse existing poles & run the cable aerially that might be only 30%-40% of the cost of digging holes, so you might be able to support a lower density suburb that way & still stick to your construction budget.
In Australia for the lower density rural / semi-rural areas they'd use fixed wireless, & finally satellite for the remote extremely low density areas where it didn't even make sense to build a wireless tower.