But for the time being it is the agency who beats down the prices, not the market.
Acencies are competing against each other too, especially for customer loyalty since most microstock agencies carry the same type of images now. Price competition is - as you state - probably much less important than convenience. The large agencies clearly have an advantage here, since with millions of shots they can cater for many customers' demands. Dreamstime and Istock have never been competing on the price level, as smaller agencies tend to do.
Ultimately, it's the photographer's responsability not to upload to sites that undersell.
Excuse me but I have one fine point to disagree with, the rest I agree with you. Look above at the bold words... I'd say that most agencies have mostly
identical images from the same photographers. That's what sets IS apart.
If someone is going to pick and choose, arguing "what is similar and what is exclusive" there is no exclusive either for those people.
Winner so far is the guy who not only had his photos on multiple sites, but also on at least three of them as exclusive (even though they were identical) and on Alamy at the same time. I got a kick out of that. I never bothered to follow up and see how many sites he was still an exclusive on?
Next down the list are the people who are IS exclusives and have a room mate or spouse that contributes to the other sites, using the same camera and lenses, with similar shots?

People can set their own prices on some sites. One of those places is SV. Another is Featurpics, where it appears customers aren't buy them in a big way, or the plan either.