I won't argue that AT&T's breakup wasn't a good thing and the right move, but unfortunately a lot of that breakup has been undone: today telephony is controlled by a small oligopoly in the US. Better than a single monopoly, I suppose, but still not great.
The Baby Bells started their re-merge back in the 90s. In 2000, when Verizon was formed, it was a combination of Bell Atlantic (New Jersey Bell + Bell Company of PA + Diamond State + C&P), NYNEX, and GTE. (I watched this all in real-time, as my father worked for Ma Bell -> New Jersey Bell -> Verizon for more or less his entire adult life.) I'm sure similar things happened in the rest of the country with other Baby Bells, though I don't know the history very well.
The break up occurred in the 80's. and since then through mergers/acquisitions, they have re-joined.
So we have monopolies again.
Somehow we have lost the will-power in government to impose the needed actions to ensure a free market. Like also denying mergers that would create monopolies.
Like how was Ticketmaster even allowed to happen. We let it buy its competitors and now it is a monopoly.
GDP? Have people forgotten that a vibrant 'Free Market' requires breaking up monopolies?
When did supporting a 'Free Market' turn into, "don't touch our giant corps".