It's hard to argue that innovation and quality of the telephone system went down under AT&T's monopoly when, during that time period, AT&T played a fundamental role in the invention of the computer and famously took measures as extreme as moving buildings while telephone operators work inside in order to avoid service disruption. It seems that other factors must have been in play as well in the eventual decline of "ma bell".
The story of AT&T's monopoly on telephone service and its subsequent breakup at the hands of both court and MCI/Sprint is a complex one that cannot be so simply used as an argument for or against the arrangement. It was a very particular situation in a very particular time, perhaps most significantly because AT&T created an entire market sector which the government had no coherent strategy to manage. So-called competition has also been quite insufficient to revolutionize the landline telephone market, it remains perhaps as consumer-hostile as it has ever been, something forgotten largely only because the cellular industry has replaced it (which, facing stiff competition but the regulatory wild west of the internet, is consumer-hostile in a whole new way).