A different monoculture is not the solution to the problem of a monoculture. Replacing all of them with the resistant ones would reduce biodiversity (perhaps the resistant ones are susceptible to other diseases, perhaps the nonresistant ones will at some point be resistant to a new disease). See https://en.m.wikipedia.org/wiki/Polyculture

The main problem with polyculture being that it assumes local farmers are smart and cunning individuals who find (perhaps unorthodox) solutions in constrained, complex and realistic environments because their livelihoods depend on it (or because they got an interesting idea and couldn't let it go), and who know more about their challenges than random hn commenters like me, instead of being mindless drones that can be replaced/interchanged at will by central overlords with complete control over the panopticon food factory.