This is exactly what I believe it's all about. So the way for TAP to handle this would be to make the "wheelchair experience" as miserable and slow as possible, discouraging pax from booking that wheelchair unless they really cannot cope without one.

The usual wheelchair process (on many airlines) is very swift, which might encourage pax to book a wheelchair only because that way they can whisk through all the formalities, bypassing perhaps hundreds of pax standing in immigration and customs queues. Apparently, your experience with TAP was very very different.