Sure, but back in those days (the 50s, 60s, 70s) two things were true about the company that bore the AT&T name: 1. They invested in research that benefited the whole industry with Bell Labs, and 2. they built out and maintained the infrastructure well.

Today they're just operating with the most cynical approach, where they extract the maximum amount of value out of a network built by our grandfathers' generation, only doing capex in the most targeted areas where the ROI will be instantaneous. (new subdivisions, and specifically ones where they can get the HOA to bake in their services to the HOA fee). I used to live in a large urban tower and AT&T did finally come in with fiber, in 2017, on that condition. They'll be cash flow positive from Day 1 because 100% of the homes in that tower are customers, and they literally can't cancel their service (or more specifically their payment).

Whether or not its monopoly was "natural" is perhaps a topic for debate. But it was definitely a regulated monopoly.