Is this easier for someone who was a "domain expert" in an obsolete domain than for a programming generalist? i.e. could a programming generalist also "own" a domain in a couple years?
My philosophy is that if PHP keeps the lights on, I better really know what it is, where it's going, what are it's limitations, because it would be pretty scary not to.
Plus once you learn a language or a database well, the next one is easier. The trick is knowing when it's time for the next one.