Are those moving goalposts I hear?
Some content can be cleared without needing 2 healers. Some content can be cleared without needing DPS. I believe P1N has been cleared with all tanks - meaning DPS is not needed (apparently), and thus NO JOB is a "-insert color- DPS". After all, CLEARLY their DPS contribution is "redundant" in terms of enrage, right? Is that really the argument you want to make here?
Not to mention that there aren't many clears without healers of any current content AND it generally takes a coordinated group to clear content without healers. Not only that, ALL current content cleared without healers is cleared with tanks, generally more than the standard number though not always. If what you say is true, ALL content should be clearable with 8x DPS, yet we never really see that. Most 0 healer clears are things like 8 tank runs.
So that does not, in fact, mean "healers are there to dps".
It means healers are there to heal, and the content is tuned low enough that 8 tanks can clear it with no healers OR DPSers if they are coordinated and know what they're doing.
So they aren't "green DPS" and tanks aren't "blue DPS.
We have RED DPS in this game. Then we have Green Healers and we have Blue Tanks.
Everyone can - and should - throw damage when their other duties do not take priority. But if as a healer you need to cast a GCD heal, that is what you do. That is what takes priority. Because you are a Green HEALER, and your DPS is secondary to your healing. Just as a tank trying to maximize a burst window that doesn't perform a tank swap because "I'm doing my DPS!!!" is a bad tank, a healer that does the same with damage is a bad healer. They aren't "blue DPS" or "green DPS" in those cases: They're bad players.