I'm not so sure all those great shows on NPR and PRI would survive on commercial radio. Commercial radio skews to the youngest listener and quite often reaches for the lowest common denominator, just like commercial television. Public radio even with it's newscasts tend to go beyond what any commerical radio network does with it's newscast. The radio networks long ago gave up trying to do entertainment programs like dramas, quiz shows, variety musical shows, concerts, etc. Public radio picked up that slack just as Public TV did.
My guess is the demo for public radio is not the same demo that listens to commercial radio. The public radio demo is willing to donate to keep their favorite shows on, because they can not find that type of music, drama, long form newscast, news analysis and commentary, interview shows, musical variety show, Car Talk, etc, etc on commerical radio. Corporations seem willing to take advantage of the ability to make a corporate donation, and certainly the various government funds both state and federal make a difference. Bottom line, I believe that public radio serves those whose programming choices no longer fit what commercial radio will broadcast, because they are not the desired demo for the ad agencies. So Public radio and TV serves that underserved group of listeners and viewers who desire something different from what the commerical networks provide. Should our programming choices be so limited that less popular forms of programming don't get the opportunity to be heard? That's where public radio comes in. Is Public radio /TV perfect, no not by a long shot, but they offer the average citizen an alternative to the usual humdrum of narrow choices that commercial radio offers. Even if you can't afford to donate, you still can hear their programming unlike satellite radio, so everyone can benefit from Public radio, if they choose to tune in.