Regulated monopolies are definitely a thing, and a valid way to protect the interests of consumers. When the regulations that constrained a monopoly from predatory behaviors are removed, as they were before the break-up of AT&T, it's reasonable to get rid of the now no longer regulated monopoly. The alternative to breaking up AT&T would have been to re-impose regulations that protected consumers, but the courts didn't have that power and the legislative and executive branch weren't acting.