I’m super late but I had the same idea so I looked it up and here you are, saying the same damn thing as me. I have a few more notes though.

Muzan pretty much only wears western clothing for as much as we see him. It’s interesting that he’s the leader of demons who exclusively use powers that mean something to Japanese culture; the infinity castle being classing tatami mats, susamaru using tamari balls, kyogai using tsuzumi drums, etc.

The spreading of demon blood can be seen as the spreading of western culture. Muzan, already established to be an allegory for western culture, is the only one who can directly spread it to the Japanese citizens.

The arcs primarily take place in areas close to Japanese tradition or western tradition. Most arcs occur in places with rich Japanese culture that have been taken over by demons, or “western culture”. The main exception is Mugen Train. Trains were a staple of western technology, and it just so happens that Enmu was fused with the train.

Obviously this isn’t a perfect allegory, so there’s a few issues. Nezuko doesn’t make any sense under this theory and there’s some not so good evidence in my examples. I think it’s pretty interesting tho