I would say that I 100% agree that the adoption of the philosophy that an appraisal had to be assigned to a specific appraiser (rather than sent to a firm who could assign within that firm) played a big part in the demise of the smedium size firm. I have explained this issue to many over the years - in fact we just had an internal discussion about this very thing last week. At my old firm, we adopted to that by replacing such clients with clients who had no such requirements. We sought out local and regional lenders who were typically easier to work with as a firm. It could still be done (I know appraisers locally who still have smedium firms), but it is certainly a challenge.
I also agree that is came with the proliferation of AMCs - but, it was not due to the AMCs themselves; rather, it was driven by the way lenders audited selection of appraisers by AMCs. I saw this when I worked at a large AMC. Lenders would come in with random list of their loans and we would have to go through the logs and show them the logic for how the individual appraiser was selected from among all those eligible for the order. That included showing how many on the panel were in the coverage area, what their service and quality scores were, etc. The lenders simply did not allow assigning to a firm - the AMC had to select an individual appraiser. I spent a lot of time trying to get lenders to change their position but most did not want to change their policies because they knew that most appraisers did not work at firms, and they saw no need to change policies to accommodate firms.