but SPM's voice does sound as if it's gone past its sell-by-date

Geez. What a short-sighted complaint. There is so much more to a song, to a performer, to the process, than an aging voice. Cases in point - Bob Dylan. The late Leonard Cohen. A couple of people whose songs have been appreciated over the years and who still have fans who love the package - the music, the words, the performer. And the music and the words will be picked up by other performers with more supple voices and performed again.

Songs aren't just for youthful voices. Like poetry they express a range of experiences. Younger critics or purists, get over yourselves.