I think a combination of 3d animation and ‘ai postprocessing’ is probably the most effective result.

As much as I respect the rights of extras, they are expensive and easier to replace than lead actors. Disney already has things setup so extras never have to be on set with your lead actors, although you get a lot of backgrounds with ‘people just walking back and forth with no purpose’, but a bit more effort will mean those prefilmed backgrounds wont even require human actors, they barely do already.