The problem is that AT&T was pretty heavy federally regulated prior to the breakup, Carterfone, etc, and the company achieved most of this work under a regulatory framework that was an early version of what we have today for the industry - and in fact because this was prior to CLECs and RBOCs was actually more similar to a "public utility" regime (e.g. electricity) than what we have today in the telecom industry, with regulated tariffs and the gov't achieving the policy goal of ubiquitous availability through tariff rate rather than the modern surcharge arrangement. The benefits of AT&T's golden era are most attributable not to their monopoly status but to their status as a government-regulated monopoly in a fashion similar to utilities operating under franchise - one which, unlike typical franchise utilities today, was permitted by regulators to maintain a substantial R&D operation.

The system was always somewhat haphazard, for example post-WWII AT&T faced 'competition' from rural telephone cooperatives under the REA as a means of speeding up rural development. However, this was initially a bright-line geographical division of duties and REA telephone coops continued to rely on Long Lines for transit.

Of course many, many things went wrong, including the regulatory regime being a cause of AT&T's decay even prior to court action against their favor. However, it's hard to say if that would be the outcome of such a regulatory framework today (or outright nationalization), because this was the nascent stage of telecom regulation which both the government and AT&T had equal hands in "making up as they went." This was the era in which regulatory capture was more or less invented, for example, and not necessarily on purpose.

My point though is exactly that describing AT&T as "a bad monopoly" or "a good regulated private interest" are not really great arguments in that both of those things were entirely true at the same time, and the particular environment in which The Bell System formed is not one that will likely ever exist again. Newer communications utilities have broadly been shoved into the category of telephony for regulatory purposes or entirely left alone, so it's hard to foresee any future in which we will have a second "telephone era" in which a new debatably-utility emerges to be managed.

As you point out, competition is better than monopoly because it provides a surplus that advantages consumers. But some things we do not want a surplus of, for example river dams. Other things can not be competitive even in principle like surface streets. Likewise as was discovered with mass transit and utilities in the last century in a world that did it's absolute best to be laissez faire.

So there are some rare cases where there must be monopolies and if you must have a monopoly, it is better to have one controlled by a democratic government than a self interested clique. At least then there is some possibility of working in the public interest on price and innovation.

That ISPs must always be a monopoly seems unlikely to me. But if it's politically impossible to break them up, then nationalizing them is still far better than the current situation.