I think there are more wheels than doors. Doors are usually on bigger objects than wheels (buildings, cars vs bicycles, chairs, wheelbarrow, luggage).
Doors are shared for multiple people vs wheels being for a single person often.
James’ point about wheels being on smaller objects on average plus fewer users suggests a sort of per capita size distribution of objects as the estimation basis… but otoh lots of poor people have homes but no vehicles.
Should we count shipping container doors? 🤔
I think you can finesse the ontological problem by impedance matching the fuzzy set boundary. If you allow bearings, hinged lids are in too. Luggage wheels in? Luggage lid in! Kitchen cabinet doors in? Drawer rollers in!