This is the idea that parasites or predators can never be too efficient as this will kill off their host and themselves. Of course this can happen, but if there are some that can survive for a while elsewhere and some hosts as well, they can immigrate into the area of extinction and be more in balance. Well, this is a hot topic of debate in Ecology and my explanation is only approximate. There are no resistant clones of susceptible American elm species. Chinese elm is resistant but is not a good shade tree.
Siberian elm is rather resistant, but has lots of twig fall.
However, I think they should plant more varieties of trees in general so that no one disease can wipe them out since most diseases can kill only one species, or a few related species, of trees. This is also true of insect pests. Usually all that can be done is remove dying trees so that less infection is present, but this merely slows the epidemic spread down so that money can be obtained to remove the dead trees. This effort was largely before the advent of molecular biology. Chemical sprays against the bark beetle are rather ineffective as the beetle breeds under the bark.