It's not about working on new content vs. adding unique bonuses to weapons.

The way you should be viewing the game is from a player experience perspective.

New content is one type of player experience.

Old content but with a new perspective is not so different a player experience.

It's not any different from any NM vs. EX fight. It's the same aesthetics and many of the same mechanics but with a new perspective.

If you add truly game-play changing unique characteristics to your weapons, you are fundamentally altering the way players interact with your old content. If you added the trait "Wanderer's Minuet no longer adds a cast time to ACH and BRD weapon skills" to the BRD relic, you would deal with a big complaint players have and drastically change how BRDs work in all existing content. If you added the trait "Sword Oath activation no longer triggers your GCD" to the PLD relic, PLDs would play very differently while addressing a concern PLDs have complained about since forever.

I think there are other issues with adding unique traits to weapons but I'm just pointing out the full impact such traits can have.

Lastly, I just want to end with a real-life example. I enjoy my music. For a long time, I just used a regular, cheap pair of headphones. After switching to a pair of much better headphones with a dedicated amp, it changed the music entirely. I was now hearing the nuance of the music that I didn't before and I instantly went to the listen to a lot of my music just to see how different it sounds. It revitalized my entire music collection.

Unique weapon traits could add new life to a stale job experience.