Games are business and what gets most people satisfied is king aka balance. Adding more jobs is not directly related to imbalance. GNB is a new job but that's finely balanced. The dmg dealt is roughly the same as the dmg of the other tank jobs so GNB is only adding variety to the tank class.
What I enjoy is having relevant DPS as a healer and SE is probably going to continue going in that direction. I don't like healing that much but I like the option of having access to heals.
This makes the strongest damage dealers the most useful healers which is why healer's dps should be greater than that of a DPS and tanks. This is how the game operates and we are getting there. Healers in HS and SB were the weakest damage dealing class in the entire game. In Shb, we are surpassing tanks in raids not only in rDPS but with sheer DPS numbers as well with our 1 button spam. In Endwalker we will most probably be closing the gap in DPS numbers with actual DPS roles if they expand our DPS kit.
We are also getting a healer that augments their own offensive spells which makes them less of a support class and more of a DPS class that also happened to heal.