Mark
I always enjoy reading your posts because they are thoughtful and interesting. This time I think your just overthinking the situation. We all know as people get older they cling to certain things and their comfort zones. The songs you mention are part of that for them. They are timeless songs that even if the audience doesn't open their mouths their singing along in their heads because they know these songs so well.
I've done long term gigs where every week the same people would request the same songs. I would think if it doesn't bother anyone else why should it bother me. I'm there to play for them.
Congradulations for keeping the job so long, they must really like you. And its a big bonus that your still thinking about how to make it better.