Earlier quoted context omitted.
People are to focused on the mono-part. IMHO appropriate laws should be applied to any company which directly or indirectly affects some "high" threshold of people. Or another way to think about it would be if it's practically impossible to enter a market even with a decent start capital because of it being dominated by a small number of mega cooperation it's not (much) different then a monopoly. What does it matter…
Having 5 companies that actually compete for business wouldn’t harm consumers. Having 5 companies that avoid competition would, and that’s called a cartel and it’s already illegal according to the anti-trust laws (the Sherman and Clayton Acts).
But if you look at markets all around the world having situations with the same harmful dynamics as cartels but for legalities not counting as cartels is not uncommon.