I'm sorry, but given the day when you posted this, I found this to be in very poor taste.

A wheelchair is not a living being. It can be propped up in a corner or put in a closet where a service animal cannot.

Also, as far as I know, there are very few people who are allergic to wheelchairs.

The OP should have spoken up at the time. They may have asked other pax (perhaps a smaller one) to swap seats with him so that everyone would be accommodated. Complaining after the fact will yield little result.