If you don't look at the public face of the companies and look at their balance sheets and income statements instead, you get a very different view of things, and they certainly look monopolistic and/or monopsonistic in some key areas. (Monopsony is the buy-side equivalent of a monopoly, where a market has only one buyer.) Advertising is a pretty significant one. An example of monopsony is that right now, while there's a crapton of places you can host your own video streams, only YouTube is an effective mechanism for monetizing them, which is why they can get away with all their shenanigans.
Once you look at it from the balance sheet perspective of these companies, and potentially from a monopsony perspective, there's a lot of avenues for a highly-active government attack against these companies, even if they never claim "the general public" is a direct victim.
* We can't because they made their services so they can't be broken
and
* They're smarter than the regulators so its no use trying.
I feel like both of those are cop-outs, and to be unchallenged makes me wonder if the two talking have a vested interest in keeping the bigs together (one as a marketing professor and the other being an equity researcher... it seems like they're not ones to push too hard against the status quo).
Point 1 seems silly - if the whole business is structured in such a way to be anti-trust proof, doesn't that mean the management is so worried about a real possibility of being broken up that they've been working to shield it? "Well they've centralized their revenue generation so there's no way to take it apart"? It seems incredibly possible to do so.
Point 2 is tactical - and that can change at any time. With the generational shift starting to happen in politics, the argument that representatives and senators don't understand the business or their products will cease to be true. I'm sure there's plenty of people in the justice department that understand it well enough as well.
Half the network is worth less than half of the original network.
Silly as it is, that's just not how antitrust law works in the U.S.