I've never agreed with any statement as to what killed prog, because I don't agree that it actually died. The late 60's and early 70's were a very experimental period in rock music, the broadest it had seen so far. Rock was firmly established and had a large enough base that it could really push how far artists could take it and still call it rock. Prog was part of that, but so was jazzfusion, early metal, and a dozen other subgenres. So prog was doing okay, but it wasn't the only game in town. By the late 70's, disco, punk, and other forms were becoming more popular. They eventually became less popular and other forms of pop music came in.
People still listen to prog, just as people still listen to disco, punk, funk, or whatever they like. It's not as big on the radio, but prog was never big on the radio. Different types of music become more popular, then later become less popular. Nothing lasts forever, but prog isn't "dead" so it doesn't make sense to try and pinpoint what "killed" it.