So first, telling me that it has nothing to do with, "...the DM determines if success is automatic, failure is automatic, or the outcome is in doubt. Only if it is in doubt and with meaningful consequence for failure do you roll." and then giving examples of the bolded portion is contradicting yourself inside of the same paragraph.
And again, the way success is determined has nothing to do with the fact that the player only describes his intentions. For no valid reason - as this has nothing to do with the current discussion - you bring this out of the blue to confuse the issue.
Second, your second example with the dagger is absolutely, 100% about player agency. If you don't have a valid in fiction reason for overriding the player's declaration, you are removing his agency and playing his PC.
First, this is NOT what the rules and the system say. So there is an extremely valid reason to override the player's
declaration. Second, I've given you a very valid fiction reason to do so, I actually have provided a plethora of these. And I'm sure that you are perfectly capable to do so yourself.
A toddler would be quick enough to beat Usain Bolt if Bolt was standing still when the toddler drew back to throw.
I'm not the only one who has told you that YOUR realism has no place in these discussion. It is only YOUR personal preference, and there is no support from the RAW, and I totally reject it as a premise for my games.
And the second portion of your response, "...,while you are still trying to decide how to throw and minding the rest of the situation and the surroundings, you have telegraphed your move and that guy over there pounces on you and kills you." is playing the PC. The player decides what his PC is focused on. Not you. And there is no "how to throw." He's going to throw it the way he has always thrown his knife.
And yet, the system has determined that he is going to act later, despite the declaration of that particular player, because there are other players involved, including the DM, and they ALSO have the right to their own agency. So please don't bring that stupid principle on the table ever again, the player is not the king of his own little bubble, he is playing as part of the group, that group is using a set of rules, and he can go and enjoy his player agency somewhere else if he does not respect that of the other players.
If you're going to play the PC like that and then just announce the PC is dead, you're abusing your authority as DM.
No, I'm not. The RAW give me full support, all you have are your preferences, I have mine, no player has ever complained about them, and your preferences are in no way superior to mine.
So again, you are welcome to screw your own games with your personal preferences and turn these games into something totally inconsistent and that causes you headaches about explaining things, but it's the result of YOUR preferences, NOT the game in itself. Stop trying to impose them as the only "valid" way of playing.
Yes, but unless there's a valid reason for overriding the PC and/or playing the character like you did in your second example, you're abusing your authority. Let's say the PC has come to a canyon the size of the grand canyon and the player has announced that his 3 int, 3 wisdom barbarian is going to jump over with a running long jump. He has no magic or supernatural abilities that would apply.
DM: "You begin running as quickly as you can and at the edge you take a grand leap out over the canyon. You go about 25 feet(20 for strength and 5 feet for a nice athletics check to go unusually far) and then plummet 1500 feet to the bottom of the canyon. Ouch! You take 77 points of damage." - this is a valid auto failure narration. It's impossible to jump across the grand canyon on foot.
DM: "You begin running a quickly as you can, but just before you get to the edge you realize that you can't do it and stop." - this is not a valid auto failure narration. You have no right to play the PC.
DM: "You begin running as quickly as you can, but before you get to the edge the NPC wizard casts Dominate on you and forces you to stop, since you failed your save." - This is a valid way to play the PC with a failure to jump, since there is a valid in game reason for it.
I'm sorry, but I don't accept you labelling certain ways of playing as "valid" or "unvalid". you have neither the right, the authority or the competency to make such calls for anyone playing the game. Some of these ways above might not be what you expect, but if they are used at a table and people enjoy the game, they are way more valid that your claims of "plausibility" of situations.
Depending on the play styles at any given table, all of these might be good or bad, it's just a question of table preference, and yours have zero value in general debates. At best you can say what you like or don't like, but it's about it.
Finally, you can say whatever you want, but once more the players does NOT know everything about the world. Maybe there is a wall of force across the jump and he WILL die if he tries the jump, Grimtooth has given many examples of that. It might not be your preferred way of playing, but you have zero right to declare that invalid.