Yes, there are a few isolated areas of the world where polio is still a threat and vaccine efforts are very important including for anyone visiting that area, and yes, it could get bad enough that anti-vaccine morons could reintroduce polio to a developed nation and that would be disastrous. However, for the time being there is no minimum herd immunity threshold for polio in the developed world, it is eradicated there and vaccines for it have not been recommended for the general populace for quite some time.