The usual argument against is that if you have "a number of specialized models" that perform well in ensemble, you can take that ensemble, and distill it into a single larger model (dense or integrated sparse, like MoE), and get the same improvement in performance with an efficiency win.
This works because having those "specialized models" duplicates a lot of the highly conserved "low level" wiring that's required for a model to function at all. As such, you end up running a small scale version of the same "backbone" computational processes many times. "Merging" those models into a larger, denser model allows for a singular strong "backbone" to be used for everything.