The problem with "rule by consensus" that I see is multiple.
First, it's simply a matter of time and flow of the game. In order to keep the game time moving I'd rather have a rules discussion outside of game time. Want to chat about it after the game? Have a discussion about house rules? Sure. Just not in the middle of the climatic fight against the squirrel of doom, please. Besides if someone is questioning there's times when as a DM (or player) I want time to mull it over and think about the options.
Sure, nothing about my examples precludes that.
In fact, I would argue that unless it is of dire importance right in that moment, rules should always be discussed outside of the game.
I think the reason you bring up this example, is because you are conflating "final say" with "Ruling in the moment" but the advice always always always is that you make a quick call now, and then look up the real rule later and address how you will be moving forward, either with the split-second decision, or with the more researched answer.
A lot of people simply don't care and they'll go with whatever option is the easiest socially. If given a choice they'd abstain. Oftentimes there can be one person who pushes their ideas on others; if you ask questions where that person does not know who is answering what the answers change. You can get situations where people simply don't understand how the game works.
True, but you'll notice I do keep mentioning that.
As an aside, I do wonder how much of that is because we always tell players they don't need to care. I've seen it where there is an attitude of "You don't need to worry about how it works, you just need to roll the dice and tell me what you are doing" which I imagine leads to a bit of complacency.
But even with simple player stuff, like planning, not knowing what you can do or how things work, can really hurt your ability to make good plans. Which makes me consider this not only a seperate issue, but one that is kind of troubling for the game as a whole.
Players think from their perspective and what will be good for the PCs at the moment. They often don't think about what it means to the balance of the game. Yes, getting a long rest whenever you want is fun, but it also makes any kind of challenge difficult to set up; in addition a lot of people don't enjoy the game on easy mode. I know I don't. I've had people thank me privately when I didn't let a single player push their agenda. The same people didn't really say much when we had the discussion as a group.
See, but these two pieces are in opposition.
You say players are only going to think about what they want in the moment. But, they don't want an easy game.
And, thinking about these responses, and how often this comes up, it starts to seem like a lack of trust in the players. Not all of them, because you had that silent group that was content to let you as the DM handle the situation, but I feel like a lot of people want to declare the DM as an ultimate authority, because otherwise the players will cheat and break the game for their own short term amusement.
Sure, bad players are bad. But if we want to talk about DM authority in the abstract sense, where it derives from, where its limits are, I don't think relying on "but if I don't have the ultimate authority, bad players will cheat and break the game" is terribly helpful.
And, as you'll see in my other post, ignorance is a thing that can be fixed. I'm sure we would all agree that a player with a strong understanding of the game's balance is very often not causing problems by proposing things that break that balance.
Sure, a bad player won't care, but a good player will realize the balance implications, and instead of proposing "I want to do X" they may approach it was "I want to do something like X, but I can't think of a way to balance it, do you have any ideas?" Which, is a much more collaborative and easy process.
Last, but not least there are many times when the DM simply understands the rules better than most of the people in the group. A lot of players are quite casual about the game (I know, shocking!) and don't really understand how the rules work or interact with each other.
Obviously a DM should try to reach consensus, I just don't remember many times when there was such a clear line.
Guess I should have read your last point, but this is also... kind of an excuse, isn't it?
"My DM authority derives from the ignorance of my players about the rules" is a very poor foundation for authority, because it disappears once the players understand the rules, which should be the goal of the group. Maybe not every rule and every interaction, but educating yourself on how the game works often does lead to more fun for everyone.