The US government accepted AT&T’s natural monopoly of telephone communications (this is the Kingsbury Commitment of 1913 mentioned in the link you provide) in exchange for their divestment of their control of other forms of telecommunication—namely telegraph communications via their controlling interest in Western Union—which was the other major important telecommunication technology at the time in the early 20th century. I can only imagine what things might be like if AT&T was allowed to keep an unchallenged natural monopoly over all forms of telecommunication and further allowed to hold that monopoly to the present day.

We already know what would happen, landlines would still become irrelevant as cellular took over. There are and were still local “natural monopolies” with phone services.