They simply get tossed.
I once found a 4x5 Cambo with a Rodenstock lens in a Goodwill retail outlet, thrown into a bin and covered in dust. It had obviously been thrown around and picked over by many people who had no idea of it's value.
Thankfully, it was rugged enough to emerge basically unscathed after a careful cleaning.
Face it, we are a small percentage of photographers who know and value this equipment. To others, it is simply "some old junk" from grandpa's house...