Hard to tell, I dont believe there is data about this. I was aware of the 2 you mentionned being a punk-rock fan. Original punk-rock values political activism and education so that would make sens. The only other PHD I can think of on top of m6 head, in popular music, is electronic musician Floating Point. Math PHD I believe. I also know some musicians like Flee from RHCP went back to university in their 50s, to study music theory. Now, if you look into contempory music, as well as jazz, it is far more common to get higher music education, in electroacoustic for example. I could name half a dozen composer with a doctorate in composition, only in my city, Montreal.