Could it be done by making a sparse MoE of thousands, or tens of thousands, of smaller experts in very niche domains? Maybe a tree-like structure of experts which can delegate from relatively general but inaccurate to extremely niche but accurate? Also these experts might be plug-and-play, easily swap out an inferior expert with a stronger one in the future without having to redo the whole pile?

That's not really how the experts in an MoE work. They activate on token probabilities and are activated on every token. You don't necessarily have a discrete math expert and a discrete physics expert. And if it were you would still need a router that is trained on all of those domains.