Another thing, you also can't just have a support role and a healer role where both are equally capable of meeting all healing requirements in the game, yet the support role does much higher overall damage. It creates a massive balance issue in which the support role invalidates the healer role entirely.

In an RPG, every role you create needs to have a defined purpose. For example, why does the DPS role do more damage than the healer role? Ignore any preconceived notions of what a job "should" offer and think purely from a design standpoint. What makes it a bad idea to have one class that deals a certain amount of damage and another class that deals roughly the same amount of damage and provides considerable amounts of healing? The reason is because in that example, the healer doing as much damage as the DPS renders the DPS job invalid because why bring one job that only attacks when you can bring one that attacks just as hard but also can prevent the party from dying at the same time?

RPG gameplay is defined by damage: the damage you need to defeat the enemy and the damage the enemy deals to try and stop you. Every role needs to interact in with that in a way that is holistically unique to that role and also is required. DPS offer the highest amount of damage which means you can win fights more quickly, but this is offset by their inability to endure the damage dealt back to them for long enough to actually win. Tanks offer the ability to not only endure that damage without dying, but also can control aggro (even if at a very basic level). If you had a team of 4 DPS in a dungeon, the only way that group can control aggro is by certain players delaying their attacks to avoid generating too much aggro and stealing threat from whichever DPS is trying to tank. Meanwhile, even with how gentle outgoing damage is, DPS jobs aren't able to endure trash pack damage for long, and even the wet noodle tankbusters of dungeon bosses will hit hard enough to kill a DPS player, or drop them to critical HP of which they can not restore very quickly. If DPS jobs could tank well enough without an actual Tank job, then why take a Tank at all and lose out on damage when the value they bring isn't necessary.

This is a very real concern that healers are met with. Even though the community is still very comfortable having regular healers, we are teetering on the edge of a future where healing requirements are so low, and non-healer sustain is so plentiful that we can very easily tip into an environment where healers become obsolete. So now let's imagine that we add in a support role that has as much healing throughput as the healers but now also has much better damage. Healer jobs will start becoming excluded from group content, and you may get people who will accuse you of trolling when playing as a healer even if the healer is still capable of clearing. If they're blatantly inferior to supports, that won't bode well for the game's balance or the community's outlook on player participation.