The thing is that we really, really want to be able to stop giving the OPV vaccine because - aside from the fact that it requires a bunch of resources and there's always the risk of disruptions to vaccination from civil wars, other pandemics, etc - the original unmutated vaccine itself paralyzes and even kills a small fraction of the people it's given to, and at low levels of disease those cases can easily become more common than actual polio-related paralysis. That's why it's not used in the developed world anymore.