That's a bunch of horse manure. AT&T was holding back the advance of telecommunications technology, because they were more interested in preserving their existing business structure. They even held back their own inventions. They invented ISDN and ADSL in the late sixties. ISDN didn't become widely available till the 1990s. ASDL didn't become widely available till 1999 - 2000. I was one of the first DSL customers in Corpus Christi and had to be on a waiting list to get it when it first became available. It is competition that forced new technologies to become available.
There used to be a joke that ISDN stood for "I still don't know." It took the growth of demand for (semi) broadband services before ISDN even achieved modest market success.
One of the problems in rolling out ISDN was that the Bell heads were scared of repeating the Picturephone fiasco of the early 60s.
To my knowledge ADSL was not developed until much later. Bellcore was working on it in the early 1990s; it was called video-on-demand at the time. Again the marketeers were wrong; they did not foresee the 'Net, and thought ADSL would be used to compete with cable.
Well if you wanted DSL back in the 70s you'd be paying an exponentially higher fee than you are now. People just expect plant and equipment to fall from trees and protest loudly when their bill goes up 50 cents.
Competition had little to do with DSL availability also. Cost-return is all they look at. If anything competition hinders it.