But aren't some of the microstock companies owned by larger companies? Seems to me there are multiple market segments and that they are diversifying to meet the demands of each.
That is correct, and the differences in image quality can go both ways, but while the technical quality is mostly very good at the micros, creativity is often lacking. I suspect one of the reasons for that is the reviewers. Most of them don't know the market well enough to understand the difference between "creative, but commercially interesting" and "strange photo with no potential".
The result is that most of the micro portfolios consist of technically perfect, but rather uninspired mainstream photos. Images that won't insult anyone, but they won't spur any strong feelings either.
Those who lose the battle are the macro-stock photographers who aren't creative enough to compete with the best, and not productive or technically skilled enough to compete at the micros. And no 48MP camera will change that. A bad photo is a bad photo, even if taken with a Hasselblad.