Many of them are brass or zinc so when the bike rusts away they remain. But for the headtube to rust away it would likely be out of human sight in the woods or something. Then the chances of someone finding the remains would be pretty slim. Of course old bikes are still found in sheds, barns, and abandoned buildings, but removing the headbadge kills much of the value to a restorer. That makes the really old ones a real challenge to collect.
On the up side, you don't have to rent storage space for a collection... or if you do you have more important things to address than a hobby
