We nearly always dress up more for dinner, certainly at Sigs, and I agree there were plenty of too-casually-clothed diners at Monsieur Paul despite its refined atmosphere.
Holy Guacamole! Just checked the Disney official menus and looks like MPaul may reopen with yet another prix fixe menu, no a la carte, at $175/adult, no kids price given. Details here:
https://disneyworld.disney.go.com/dining/epcot/monsieur-paul/menus/
That scares me regarding the possible Deluxe dining plan price per day whenever it becomes available again!