A doubt I have is that this borders on a "no true scotsman" argument. Or, perhaps more acutely, what is the importance of "DM always says yes" given players are expected to self-regulate so that no instance could arise in which "no" would ever seem justified?
Good question, and I would say it's because to me there are two different types of 'No' we are talking about. There's the 'No' from the DM who just shoots down a choice or an idea because it is logically impossible to accomplish within the confines of the physics and fiction of the game world... and the 'No' that results from a failed die roll.
A failed die roll is usually a 'No' response to some choice being made.
"Do I hear anything meaningful inside the cave?" (die roll) "No, you only hear the wind."
"I wish to convince the guard to let me into the shop." (die roll) "Sorry, the guard says the shop is closed and you may not enter."
"Can I take that purse off of the highborn gentleman without being noticed?" (die roll) "Unfortunately, the coins in the purse jangle as you try and he looks down at you just as you were trying to swipe it."
These are all scenarios and choices a player can make in which a 'No' via a die roll is a justified result, and there was no self-regulation needed or required of the player... because the choices and questions they were making were all legitimately possible. But because the choice
was possible... this particular game paradigm just says that it's
always accomplished (whether we say it's because every die roll just happened to succeed, or because the DM skipped the die roll altogether and went straight to the positive outcome result because they "said Yes".)
Basically, you play D&D like you always would, except that any time a die roll would have been asked for in response to a choice you made... the "die roll" just happened to succeed each and every time.
My comment on "self-regulation" was only needed to counteract the idea some other have made which was that they believed that
all players would intentionally ask to do the impossible because they knew they could "get away with it" in this particular game paradigm and thus break the game. But I believe that's not the case-- that there would be some players who could "self-regulate" and not "do the impossible" all the time because the game without the DM telling them 'No' allowed them to.