Is the infrastructure for low-broadband in low density areas inherently more expensive than POTS infrastructure was 75-100 years ago?
No? The process to transition the USF to covering broadband didn’t even start until the tail end of the Obama administration.
An example, in Carroll County, GA I live outside of Carrollton, GA and have 2 fiber options 1) Charter/Spectrum which I use 2) CarrollEMC/Crossbeam which looks like they just dropped fiber on our power pole within the last month. If you look at the FCC map of Highway 5 it correctly shows ATT DSL availability, incorrectly shows Comcast coax availability, and does not show either Charter/Spectrum or CarrollEMC/Crossbeam. There is only a process to challenge availability on the FCC map, like Comcast...that incorrectly shows availability, and no option to add new providers to the map. It is left up to the provider to claim availability.
There are now plenty of rural areas with symmetrical gigabit broadband available, while most cities are stuck with whatever pitiful options Cox or ATT decides to bestow.
I don't know why. It seems incredibly realistic to expect wired broadband in most rural areas. (We were able to provide them with power lines and telephone lines over 60+ years ago, there's no reason fiber would have to be any different)
I'm not sure what the answer is in general. It's probably not realistic to expect universal broadband in very rural areas. Conventional satellite is by all accounts pretty bad. And cellular isn't really an option absent a good 5G signal.
But, other than a vacation cottage, lack of usable Internet isn't really an option either if you're going to be working from somewhere.