Right, but there are literally hundreds of tiny nuances across the books. You may never have had to change d&d 1st edition - i did. you didn't have to change 2nd edition - i did. I changed 3rd, and 3.5, and 4th, and 5th. I've changed forgotten realms, i've changed dark sun, i've changed ebberon. There is always something that gets missed.
I've completely abused almost every rule system - in d&d, I made characters that had over 1000 strength, and which had hundreds of thousands of hit points. No rule system works the way you want it to - if you think so, you just haven't pushed it far enough.
But, to be clear, in my opinion, if an author says 'huh, i like where this world is located, and its population is 6, but i want my story to be 500k people' and then simply says in the description that its 500k population, that's NOT using the flex rule. That's the author making a mistake - they should have either found a different world, or changed the pop code to match the description. That's very different to what I'm talking about. An author, should have the time to prep in the way I'm suggesting to you. And when authors make those kinds of mistakes, its fine to point them out. That doesn't change the necessity and reason for the flex rule to exist.