That's more or less what I'm highlighting here. It doesn't change the supply vs demand
The demand is for dungeon DPS slots. Going from 2 to 3 would increase the supply by 50%. That other factors may limit the impact of this change does not negate that this would increase supply by 50%.

Consider an extreme where it takes 1,000,000 DPS players per 1 tank and 1 healer. Supply would far outstrip demand.