From my experience, I can say that all commercial flights do not monitor 121.5,
but enough of them do that a downed or stranded aircraft would have a good
chance of contacting one.

By the way, did the FAA lift the requirement for monitoring 121.5? If they
did, I don't recall seeing it. Even now, on 121.5, I still hear ATC calling
aircraft with whom they've lost communications and to warn aircraft about
possible airspace incursions .

Mark