I honestly have never had this issue. It's really about focus and effort. If the person is important enough, I will most likely remember how to contact them. I also mostly use email for all important communications so that remains searchable. The other chat apps are all for casual friends and acquaintances that if lost, well so be it.

None of my older relatives use email. They can text however (or iMessage, since it is transparent). Not everyone cares so little for friends and acquaintances.