My experience has been that the editors of Math Reviews pay attention to suggestions about revisions of the classification system. I'm not saying that they implement all the suggestions (especially because the suggestions sometimes contradict each other), but they do listen and, a while before the once-a-decade revision of the system, they ask some people (including me on some occasions) about both the proposed changes and any other ideas we might want to contribute. The revision process is non-trivial, partly because of the need (or at least desire) to coordinate with Zentralblatt, and partly because a major revision of any section has a big downside (the loss of backward compatibility) and must therefore have a big upside to make it worthwhile. But big revisions have happened, and they may well happen again. The editors also watch for classification areas that have gotten either unpleasantly small (so they might be merged into other areas) or unpleasantly large (so they might be split, if natural dividing lines can be found).