The U.S. could become a mission country as well, and our clergy would be comprised of a large number of African and Asian nationals. There is not an overabundance of African or Asian priests either.
I would wager that celibacy would be made optional long before the Western church is allowed to lapse into a missionary status.
American Catholics aren't going to long tolerate suburban parishes visited by a priest once a month.
Dunno about a 'shortage' in Africa. We have a number of African priests in SE WIsconsin, some of whom are parish administrators/curates. SOMEBODY has to be authorizing them to come here...