Ah, that is really triggering another insight. There are flexibilities on different timescales. My statement was with timescale of within a single session, while your argument is in the timescale from one game to the next.This is quite strange to me, because introducing a rule into a set of rules saying "you can change these" doesn't actually make the rules in question easier to change.
AW has a whole chapter talking in detail about how to add to or change various aspects of the game, and obviously is very successful in this respect given the number of PbtA games.
PbtA games tend to be quite rigid with regard to how a spesific session look like, but it is easy to make large scale changes keeping the core concepts allowing for a very wide range of possibilities when changing around rules for a new game.
Rule 0 games (including D&D) tend to be very flexible in terms of the width of experiences that can be had within a single session. But I fully agree that D&D is very rigid in terms of making major changes to the existing game as such between campaigns.
I find this distinction fascinating, but I don't immediately see how malability of game rules from one game to the next tie into the main topic of this thread - DM control? For instance i am of the impression that gurps still is very malable (on the cross game scale) while granting the same controll to DMs as D&D (hence leading to similar in-session flexibility)