strictly speaking, a monopoly by itself is probably not a positive externality, since it isn't external, other than the regulatory permissiveness that allows it. to me a vertical monopoly isn't necessarily a single business, even if the stack as a whole principally delivers a single type of good or service.

if a layer in the stack can thrive on its own as an independent ecosystem, then that's a "separate business", and trust busters should look askew at conglomerations across such boundaries. independent companies will make the whole industry more efficient in the long run, because of competition. in such a configuration, companies can make deals with each other across stack boundaries and even invest in each other as long as a controlling interest hasn't been reached.

ma bell for instance vertically integrated telegraph, telephone, transmission, real estate, consumer phones, answering services, and who knows how many other whole businesses together to form its monopoly. many, if not all, of those could have been thriving markets on their own.