I think they only "eat"ash. Meaning that if all ash is gone, they'll go extinct. In principle. But since that is not a homogeneous process (pockets of ash will survive longer, sustaining the beetles, which can then spread to any new ash elsewhere - if that (new ash) is even possible). It is near impossible to get rid of invasive insects in a situation like this. (Unless another invasive insect is introduced that *solely* hunts these beetles. And if that solely is not 100 pct solely, another species of insects might be wiped out....)