The good news here is that virus genomes are usually so small and parsimonious that they find it hard to cross significant fitness valleys between different loc

The good news here is that virus genomes are usually so small and parsimonious that they find it hard to cross significant fitness valleys between different local maxima. But they also have monstrous cohort sizes, so it's not impossible for them to get "lucky." (Lucky for the virus in a Darwinian sense, unlucky for its host.)