> vertical (not horizontal) acquisition [..] any acquisition or merger would be a monopolist’s act

Past a certain size, it absolutely is. E.g. a food producer will have a much harder time entering (or staying in) a vertically-integrated market (as an independent entity), where all the grocery chains privilege offerings by producers they own.

Using other near-monopolies as argument for why new ones should be allowed to form will lead to a very consolidated market, which is exactly what anti-trust is supposed to prevent.