I don't think it should be an issue really!
If any new researchers pick up a Rootschat post on one of the old threads, perhaps through a Google search for a person, as I did years ago, (and promptly joined up) even if the original poster has not been online for years, (given up, or maybe passed away) they may well find some great information that pertains to the family they are researching.
It may even just be one little fact that leads them to some helpful family information, and they will probably become a Rootschatter themselves!
I thinks it's great that they are kept visible, and can still be added to for future possible researchers!
After all, none of us can post that we are seriously ill, in hospital - or have died!
Sarah's got it right!!