I think the FIM and BSI want to get away from the possibility that the young Poles will come to dominate the GP's, which would be great for Poland but not for world speedway as those 2 bodies might possibly see it.
They know that once Polish domination is total, interest from audiences around the world will diminish accordingly.
The problem these bodies have, regarding young riders from other countries becoming good enough, is the reason you gave and it will be interesting to see how they solve it. Perhaps they could sponsor them themselves.