I think YABU, as you ask.
Nobody has to buy a costume, my DGC have never had a ready-made one yet. And it doesn't have to be over the top either. At my school we used to have a few piratey scarves, cloaks, crowns and suchlike to hand for children who didn't come in outfits but wanted to dress up, so noone missed out. DD2 is a bit flummoxed this time as DGS2 wants to go as a dustbin. From a page in his favourite book which is all about useful vehicles (he hasn't quite grasped the concept of favourite character).
There are some great free books for children to choose from this year. I'm hoping one of mine will pick Nadiya's Bake Me A Story, but I know nicely that the Marvel Avengers will win through as usual!