D:OS2 is not the only turn based RPG to have a party limit of four. What a weird accusation.
Also I didn't realize you needed a healer and a tank for a DnD group. I should go ahead and delete my current game where my party of all ranged DPS/control have been having an easy time of it so far.
Should also go and talk to Wizards of the Coast about adding tank mechanics to the game. They've clearly left out the taunt feature.
That being said I don't fundamentally disagree with an option for bigger parties. Especially for multiplayer as I'd rather cakewalk through the game with five players than have four friends and have to tell one of them they can't join.
I just think the logic in the OP is lacking.
Tank mechanic? You mean the "taunt" ability?
Contrary to WoW, in DnD they did in a smarter way : cover system + attack of opportunity and other warrior feats that allow to protect the nearby mage. BTW, for some classes they do have "taunt"