Are my concerns about the Pope valid?

You are wrong.

The papacy is a very demanding job and when you consider that popes like Benedict and Francis are already old men before they came into papacy... is it really fair and reasonable to expect them to continue with the role up until their death???

Putting aside the papacy aspect of the job for a while, these popes are still men. They will have age related degeneration of their bodies in at least one way or another. Sometimes, maybe their legacy is to change what they can change for the Church, then step aside to allow someone who is better able than they are to carry the Church into the future.

Rather than judging them for not remaining pope for the rest of their lives, I try to accept that God and Jesus are working through these popes to give them the strength to change what they can and the courage to say when they can't do anymore and step aside for the new pope - whoever he may be.

I can't imagine that it would be easy for any pope emeritus to go from being the pope and constantly in the limelight to becoming pope emeritus and basically living as a hermit somewhere in an attempt to not draw attention from the current pope to yourself.

I agree. Compare and contrast the Queen of England, in her mid-90s and in naturally failing health, but continuing to do her job, with appropriate delegation, as she promised to do when her father died 70 years ago.

Popes should stay as long as they want, if not they can go and retire

That is good that they step down if they know they can't keep up with the duties required.