Could some still work but be relegated to desk duties?
Sure, but that's really a totally different job. Some have retired and gone to work for CASA, ICAO, even Boeing. Most of the line pilots aren't keen on having management pilots who don't currently fly...they end up well out of touch, even if they don't think so themselves. In any event, there's a very limited number of such jobs.
SOs are Second Officers. They don't fly the aircraft in the critical phases at the start and end of the flight.
Whilst none of us like it...as we get older we are not necessarily as good as we once were. Cunning, knowledge, and experience, makes up for some of it, but there will come a point at which the FOs are more on top of the game than are the Captains. They should be gone before that time.
Sim sessions don't really pick up on this...remember they're the blokes who have the most experience in the sim too. In my experience, the blokes who see the issue, and leave are the ones who aren't the problem. People who either won't admit they're slowing down, or just don't see it, will always be an issue.
Basically the jet is going to get where it's going at about 1,000 feet per second. It's best if the pilots get there at the same time, or earlier...