That was the excuse used for WAR as well, back in Heavensward. That problem wasn't WAR being too powerful, oh no, the problem was that PLD and DRK were too weak.
The crux of the issue actually went back to ARR, and the fact that the game only launched with two tanks and two healers. When there's more jobs present, you're obliged to spread out different unique mechanics across several jobs, like pet based healing, shields, aetherflow's instant cast effects, and so on. But they just put out the standard 'vanilla healer' you see in every game, and the second job was 'everything else'. The same is true for tanks as well. So when they tried to add in a third, there was no design space to expand out additional jobs in. DPS never really suffered from this problem as a role because there were so many more jobs at launch, setting aside the fact that BRD was mandatory for its resource buffs and a subrole had to be created around it to make it work in the long run.
The biggest problem is resistance to change from the playerbase. The game would be very different had all the currently existing jobs been present for the same amount of time. But adding new jobs upsets the entire balance, especially when you have fewer jobs to start with, and you sometimes have to make radical changes to keep everything balanced. But the dev team seemed to be overly worried about hurting anyone's feelings, which lead to them backtracking on much needed balance changes and letting some jobs remain unchecked for way too long.
