Because Blizzard has never stated as such. The "balancing nightmare" pops up anytime there is a discussion of new specs and new classes being added to the game. However that argument only comes from forum posters. Blizzard has never stated that adding a support role would be a nightmare to balance or break the game. Saying as such would be a good way to remove talk of such a role permanently.
You answered your own question. In the end, a support role is really just boosting the existing abilities of the existing specs. If anything it could justify a ramping up of the difficulty of old boss fights since the added support would be giving groups more DPS and HPS.And how would that work for older content? Sure level scaling usually makes older content irrelevant yet even now, Mythic BFA raids can be quite difficult to solo.
I don't know how it would influence the population of the game. I do know that it would add more depth and complexity to our classes and group content, and I could see that being attractive for many players, whether they're playing or no longer playing. It would be a massive shake up to WoW, which I feel is definitely needed.That's usually not the problem. The problem is justifying the cost. Is adding a support class (along with readjusting the rest of the game) going to bring in more players than it will cost to implement? Are we going to be able to see population levels return back to Wrath days with this change?
But Activision Blizzard has tons of money, yes that's true but they still have to justify the cost to their shareholders. So instead of devoting additional resources to <insert support class> maybe they invest those resources in... Shorter patch cycles, more content, bringing GPT AI into the game, etc etc.
Well that's the point; Just like you wouldn't enter a raid without a tank or a healer, if this were to ever happen, you wouldn't enter a raid without support either. It would be just another part of the puzzle that makes a strong and complete group.See my previous point about scaling - in short, a flat buff amount (even only at specific frequencies) can multiply the remaining DPS output by such an extreme amount that you would be handicapping your group without said buff class. Going to a scaling modifier so that the boosting effect goes down as you get more players would also be difficult to balance as toss in additional variables such as healer to DPS ratio. Or healers that act as DPS (i.e. Disc priests) or what happens to the scaling when you have more than 1 buff class in the group.