> antitrust often un-locks markets and opens them to venture investment.
That's true with a caveat that the unlocking isn't for everyone but for specific members of the "investor class".
> To some extent having a market dominated by a couple beached whales is terrible for the investor class.
That's also true with the clarification that there's no homogeneous "investor class", redistribution of assets within that class is what moves the world today.
Also, while a couple of whale spots are definitely not enough for the number of candidates, too many spots are even a bigger threat, so you rarely see antitrust action as a means of opening another spot on the whale beach. Besides, there are other options for doing that, antitrust is the last resort.