So just because we're not forced to post a mod here, we give up our copyright? Anyway, if that were implemented, I would almost guarantee vB.org would lose a lot of coders.
vBulletin.org is a private entity. They could make it a condition of posting a mod here that if "abandoned" you, the mod author, gives consent for another coder to take over the mod. Since the mod can't be commercial to be posted here anyway no money is lost by anyone, but the community benefits.
You yourself worked on the My Smilies mod which went from coder to coder to coder and will probably go to another one for 4.x... This is all well and good that you or someone before you was able to contact the original coder to get permission- this isn't always possible and not always practical either... people move on, people even die... I wouldn't want my mods to sit unupdated if they could otherwise be so if I died unexpectedly.