Blizzard is making the game for you as much as it's making it for those who you claim to be mentally unstable.
If there is a demand by a large portion of the player base, meeting that demand is only the logical step to take by the developer. The reasons behind said demand being created is irrelevant to blizzard because as you put it, blizzard is not a licensed medical professional. ( And I can assume neither are you)
Now I personally think all content should have at least one queable difficulty. Locking people out of 3 dungeons is not a smart move imo.