I agree with you 100%. Those who chase achievements need a way to show if off. So when they get their AOTC mounts, CE mounts or whatever, they want players to gaze at them in wonder.
But, as content because legacy, the restrictions on rewards needs to be consistent. Those who did AOTC Archimonde in WoD were able to “show off” their mounts. While now, I never see anyone with it. So having someone else get it now would not have a negative impact on me who earned that little Moose. This applies to all rewards across the board. They at least have the achievement to show they did it during the current content.