The only problem with this, is that books are, on the whole, really bad about telling people that the necessity in traditional games to occasionally step in is not the same thing as doing it any time you feel like it. That distinction matters a great degree to at least some of us, and it gets remarkably old for people to act like the difference doesn't matter.
Just to do some levelsetting, I think the rules are a shared resource so that everyone is on the same page, especially about what their characters can do. I don't want to ignore it willy-nilly, and when running a game like D&D 5e I have two houserules (inspiration can be a reroll, drinking a potion yourself is a bonus action) and otherwise play by RAW. I'm more towards "Rules as Law" except where the rules tell you to modify themselves.
But how much changes are acceptable from that baseline really depends on the table, the ruleset, and the actual change occuring.
To stay with 5e for some examples I adjust monster CRs and homebrew monster features practically every session. Some see that as rules changes, some don't. I don't, or other like additions like making up a custom magic item. But even if I did, the DMG has rules for customizing monsters so it's part of that.
I will occasionally make rulings like "because you are in leg shackles, you have -15' to your speed" or "in these choppy seas, if your armor gives you a stealth penalty it's also going to give an athletics penalty to swim". These I feel are in the spirit of Rulings not Rules direction of that game where the "physics engine" of 5e does not try to cover all cases and leaves it up to the DM. How much stomach the table has for changes like this can vary, but to me this is one of the benefits of a human DM over a video game.
I don't make fundamental rules changes, or if I did it would be part of the pitch for a new campaign before session 0. "This is Dark Sun, there are no divine classes available, but we have several new races and psionics."
For something like the PbtA game Masks: A New Generation, I consider the Agenda and Principles inviolate. But it has a whole chapter on Custom Moves so adding a new move for a situation or for a villain is something I consider inside the rules.
Again, even with the changes above, I consider myself much closer to Rules as Law on the spectrum - but for me that includes following the rules own guidance on making ad hoc rulings and creations.
I definitly can see where GMs might go overboard, where that can degrade the shared understanding the rules bring to everyone at the table or just wander into less fun. But I would think that would come from either making changes that the rules don't encourage, making enough rulings that the game doesn't feel consistent - the same thing works differently for no good reason, or by making such a slew of changes that it effects the fundamental feel of the game.