Outside of the Relic, there are no other rewards promoting replaying old content. You have Wonderous Tails and that's about it. Other than that there is 0 reason to play old content outside a few gear pieces for glamour (With Trials also getting the ponies/birbs/wolves etc)
Introducing something during an expansion (Since Relics don't seem to be released during X.0) that makes older content relevant (Meaning, increasing the amount of relevant content for players whom have caught up with the most current content, as well as improving the numbers of people running old content so that newer players still have good queue times) is a good thing and something that some players like (Some like to complain though, they don't like doing the "Same thing again". They only like it when they do the same thing, but with a different backdrop

)
This has been the Relic in ARR and HW. Sure, they could have added in Mounts (Like the Tank mounts that get people to play Roulettes which involve old content) or Glamour. But they chose to create Relic weapons for this purpose.