OK, I'm going to try to address this, I hope I'm understanding you. Yes, I suppose it is possible with "rule 0" that a DM could ask you to do jumping jacks rather than rolling dice, but I don't see that happening. https://twitter.com/MatthewCmiel/status/1336497450324901890

Having a rule 0 doesn’t “negate the game”. It gives you the license to adapt the game to your needs and tastes. Some people do this anyway. “Rule 0” tells you that you can. This is important as I’ve seen many people over the years suggest that you shouldn’t change the rules...

... of a game as then you won’t be playing that game anymore. So some groups play a game they end up disliking or abandoning a game they would otherwise like but for a small set of rules. That’s ridiculous. D&D is a huge game, you can change lots of it ...

... and still have a perfectly enjoyable experience. Why not codify that into the rules? To my understanding Gygax never referred to a “rule 0”, he did however state that the DM is the final arbiter of the rules. He gave a LOT of suggestions on how to implement the rules...

... but he also realized that every campaign would end up different to some degree as individuals would interpret the rules differently, use different optional rules, create their own spells, monsters and items, and come up with innovations he didn’t even imagine

Saying that the DM is the final arbiter is a way to solve a problem. The problem is that it isn’t possible to anticipate every rules question that will come up in a TTRPG. TTRPGs are huge, open systems with almost limitless scope

You simply can’t plan or have a rule for everything. Making the DM the final arbiter allows you to create an on the spot solution to a problem in the game. How you choose to get to that final arbitration is not specified

Why not look at examples of the rule in context as it was intended to be used and decide if it’s worthwhile? The rule wasn’t created so DMs could be tin-pot dictators, it was created to keep the game moving.

So to give a quick example, in a recent game the players tied a rope between two trees and made it invisible to trip up a pair of charging giants. There are rules in AD&D for noticing invisible things and for tripping...

... but the context of this example made it uncertain. For example, there are rules for monsters/NPCs noticing invisible foes, but this assumes that the foes have a smell and make sound, or leave a “tell tale shimmer” in the air as they move.

So on the surface this is an easy one, just apply the rule, right? No smell, no sound, no 'tell tale shimmer' due to movement, so no chance the giants will notice the rope and avoid it. However, what if the giant noticed that some of the foliage was bent/disturbed...

... along the length of the rope? What if the wind made it move? There are no rules here. Should I create a new rule for "noticing invisible ropes", or switch to a game that has them? No, I just make a ruling on the spot , and if it comes up again try to be consistent.

If the giants didn’t notice the rope and were tripped, what happens? AD&D has rules for tripping in the trip spell, so it's easy! The trip spell states, “Creatures moving at a very rapid pace (running) when tripped will take 1-6 (d6) hit points of damage and be stunned...

... for 2-5 (d4 +1) rounds if the surface they fall upon is very hard, but if it is turf or non-hard they will merely be stunned for 2-5 segments." So again, we have a rule in the game, and this should be easy.

But what about giants? Should they take more damage if they fall because they are larger? What about the small trees and rocks below, if a giant falls on a small tree would that do more damage than falling on grass? Could they be impaled?

The spell also mentions that “Very large creatures such as elephants will not be affected by a trip”. That seems clear, giants are big like elephants, so the rope shouldn't work. But if the rope was high enough and the giants were charging, why wouldn’t they trip?

Is the spell the same as the rope? The point here is that the rules aren’t going to answer this in any clear and decisive way. Rules need to be interpreted, and in some cases, created whole cloth, to deal with situations that arise in the game regularly.

You have suggested we should have rules about how the DM changes rules or introduces new ones. How would that work, other than the standard platitudes of "be fair", "what's good for the goose is good for the gander" and "try to find similar cases to use as a guide"

Gygax was wise enough to realize that the number of possibilities here exceeds any simple rule about HOW to change the rules. Instead, he told you that the DM has final say in how to do this, however they choose to. That can involve player input or not.

This isn't bad game design, it is elegant game design, it allows you to keep your game moving despite the fact it has almost endless possible rules interactions that you can't predict and that aren't "obvious" from the rules as written

But what if somone shows up for a game and the DM has made a major change thanks to rule 0? Take an example. In a D&D game, rather than telling PCs their HP, and how much damage they have taken, say the DM just told them how they felt, e.g. fatigued, in pain, bleeding, etc.

For many, this would "not be D&D" anymore, as reporting HP damage and telling players their PC's HP are well established and reliable parts of what most people would consider to be "D&D". Rule 0 says you could do this. Would it still be D&D?

In this case it would be, because "described damage" is an optional rule in 1e AD&D. But most people don't know that. So when I have suggested it, many people have suggested that doing this "wouldn't be playing D&D anymore".

Rule 0 is a tool to make your game flexible and fun, it allows customization and adaptation, and in TTRPGs this is a must. It doesn't "negate the game", it empowers it. Could it produce a very different D&D game? Sure, but so what? If it's too different you can leave

If it ends up being fun you can stay. I realize that the prevailing wisdom is "if you have to change D&D you should just play other games", but I think this is needlessly restrictive. Giving the DM the freedom to be final arbiter was a brilliant game design decision.