On the one hand artificial scarcity sucks but on the other hand people's time and investment should be respected. If you're told a glam is unique to a one off piece of content then it should be unobtainable after out of respect to those who went to the effort of getting it. The issue I take here is the content having been a one off, it should be recurring content with a new/tweaked glam reward so if you want the 2015 edition of the glam you have to do the 2015 event.

As to the question of gatekeeping mounts/minions etc, at most I would argue that to get achievements and titles you should have to do the content synced, I would also accept drop being raised (as they are) as content ages but that they should be lower when unsynced to incentivise people doing the content as intended.