I'm not sure the concept of a monopoly/monopsony works against us.
It would work against the client. If they only had one choice of MSC they'd have to pay whatever The Blob charges or forego the service. A monopoly MSC is going to charge more for reports. But for us it doesn't really work that way because each shop is unique and their needs are much less flexible than ours.
You've never been able to choose between MSCs for the same job. If I like MSC "Plan9" and I hate MSC "BlackLagoon", it's not like I can say "Sure, I'll shop at Alice's Diner but only for Plan9." I either do it where it's offered or I don't. If Alice's Diner switches to The Blob, again I either do it where it's offered or I don't. Making the MSCs compete for me was never an option.
Also I don't know about you guys, but I don't do this full time. In a perfect world that would be an option for us. But it's not and I don't treat it like one, so I don't scrape the bottom of the barrel for every crappy listing. Meaning if I decline a job I'm not suffering as a result. But the MSC still needs it done. So they don't really have pricing power over me.
That said, anyone who really needs the money and has watched opportunities decline over the years, you have my genuine sympathy. And I recognize French Farmer mentioned gas stations specifically, which people do regularly route. If you're in a spot where you need the job no matter what, they do have pricing power over you. But my first point still holds - that would be equally true with 3 MSC each lowballing you and the consolidation is irrelevant.
To the contrary, what we should probably fear more is the opposite: MSCs competing against each other for clients by lowering their rates. The less they make, the less they can possibly pay us under any circumstance. If your goal is to shop Alice's Diner, you should be rooting for The Blob to demand $100 / location, rather than Plan9 charging $15 and BlackLagoon trying to steal the client by charging $14. In this particular sense, they are our ally and we should want them to make money.
Of course none of this prevents The Blob from being incompetent. That's annoying to see. I just don't see "monopoly" being the risk here.