since it would require overhauling the way loot tables work.

They've changed the loot tables before...this is a cop out excuse. On top of that...they only have to make the change in the expansion/LW maps that are affected by the new elite specs, no reason to change older content.

totally new animation sets, makes it a lot of work.

This...this one is the real issue. Except...they have to come up with new animations for classes getting new weapons anyway. For upcoming expansions and elite specs...it's not an excuse, but this is why they won't add anything retroactively. With elite specs getting access to new weapons...they absolutely can since they have to do new animations anyway.

For the most part...new weapons does come with some new animation work but it's largely done creating a new elite spec anyway. I honestly feel like the GW2 mount system was more work than new weapons would be. By and large...I'm okay with this since mounts were planned with HoT but eventually set aside to due "the tech not quite there yet". New weapons are possible...but it will take close to the level of work for the mounts we now enjoy.