"Before you jump to any conclusions about these companies, though, remember that the turn of the millennium was precisely when a lot of modern Internet cabling was being rolled out."
I feel like this is a very important point that is getting lost.
Of course costs are going to be higher and profits lower (or even negative!) when you're laying lots of new fiber and deploying new equipment. Maintaining that equipment in subsequent years will cost much less and you'll make more money those years selling service on it. There's nothing inherently wrong here.
My guess is they were barely loosing any money in the years they upgraded their network and have recovered their investment within a few years while raising prices even more under the false pretense of needing/wanting to upgrading their network even further.
It would be interesting to have a line on that graph showing when ISP's weren't required to abide by FCC rules because they were not classified under title 2.
Yet with all this purported modern fiber, only like 5% of the US population at most have access to speeds over 100MB/s, and only at exorbitant prices that do not reflect the current low cost of maintenance of that infrastructure deployed a decade ago (when these companies were, note, still profitable).