Once you go to digital, that won't be much of a concern. It will be looked after automatically. (sorry)
I think we owe people the access to these photographs/negs since we are sort of the keepers of the family archives. Personally, I've never thrown away any negatives in 30 years. When I sold my studio, the negatives went with them. As for the last 20 years, I still have them.
To me it's simply a matter of communication with people how long you will keep them and that's your decision. At that time I would guess you should contact them and give/sell them to them.
As for people that have moved etc. it's not my fault. We're not the National Archives. As long as you do your due diligence.