I'm going to lump for your first two responses together since we are specifically referring to SC with opposing antagonists.
I never made the claim antagonists always come into play but unless you are claiming that they never do in a SC... I'm not sure how this addresses my stated issue?
So wait there's no uncertainty in an easy DC? An easy DC in 5e is 10, a character with an average score with no training has a 45% chance of failing that irregardless of level... even a trained 20th level character with no ability modifier bonus has a 15% chance of failing an easy check.
In other words, even presupposing a mid to high level party...Wouldn't your above assumptions depend on who is making the check (trained vs. untrained/high ability score vs. low ability score/expertise vs. non-expertise), especially with bounded accuracy involved? So I don't think you can dismiss the situation with the party, for the most part, succeeding just because the party is high level... I don't think the assumptions you are making here are necessarily correct.
Regarding antagonists I'm suggesting that in a SC one could use passive DC's based on the skills/abilities of the antagonists - as opposed to rolling for them. Regarding the rest of it, I don't disagree with you at all, I think my initial post dealing with this issue was unclear.
Not sure how this apples to oranges comparison applies. In other words me designating the hit points or number of opponents does not in and of itself place limiters on how the PC's must deal with said opponent... whereas if I say the encounter must last X rounds (irregardless of what the PC's do) and they must eliminate Y hit points before it ends (eliminating such actions as running away or negotiating, etc. that could end the encounter before the prerequisite hit points are loss)... well that's a different can of worms.
You initial contention was that you dislike mechanical process supposedly dictating the fiction.
There is nothing stopping a party from refusing to chase a thief and thereby leaving a SC.
There is nothing stopping the party from refusing to escape a collapsing tower and thereby leaving the SC.
And I find it totally different as I don't set predetermined conditions on how the encounter must be interacted with before the encounter begins. I don't set the number of attack rolls that must be made successfully before the interaction with the encounter is a success... there may not be a single attack roll made if negotiation, trickery or even intimidation are employed...
Okay so I lean towards player fiat (and I'm by no means an expert in SC), but should a player derive another method of achieving a success without the use of a skill (which is highly plausible as the DM cannot think of everything), I count it as a success. Granted, this is not the mechanics of the regular SC but the goal is the same we are still attempting to gain x successes over y losses.
And again this doesn't address my issue. The SC structure in no way enforces that the narrative must actually create a solution to the fictional challenge... only that X successes must be achieved to enter a "success" state before Y failures cause a "failure" state.
Yes but I am not going to narrate actions for characters that haven't been taken to force a narrative conclusion the mechanics didn't create (Because now it really is just a series of dice rolls).
You don't have to narrate action declarations, however the goal of the SC should be established beforehand - whether it be to journey to Bryn Shander during the heart of winter, escape a collapsing tower, catch a thieving culprit or obtaining an audience with the princess. The party's previous successes propel the outcome of success.
So in the SC example attempting to gain an audience with the princess - lets say its easy, so 4 successes before 3 failures
1. Attempting to gain an audience with the princess using normal channels (failure but you might get a lead about her closest subjects)
2. Discovering the princess's personal handmaiden and her daily route - through investigation, bribery or persuasion (1st success)
3. Making a good first impression with with the handmaiden at the market square - whether is be through general charm or an impressive display (2nd success)
4. Impressing on the handmaiden that she must deliver the note your wrote to the princess ensuring her of your good intentions (3rd success)
5. Using the manor's map given to you by the handmaiden you decide how you wish to enter noble manor and to bypass the guards - (4th success)
6. Perhaps the door to the princess's room is being guarded by a dog - there would have been another check to pacify the dog by throwing scraps of food or even casting a spell, but because 4 checks were already achieved - the handmaiden is narrated as being there to calm the guard dog and so no alarm is raised.
Honestly as a DM this is a pet peeve of mine, I don't narrate what my player's characters do while they just roll dice.
I find this statement strange. Not even in combat? And they don't just roll dice, they provide action declarations to DM scene framing before the call to dice.
My players interact with the fiction and use the mechanics when there is uncertainty as to whether they can achieve the results they desire. If I have to step in and narrate how all of these skill checks and fictional actions they've taken actually come together to provide a solution to the challenge... well then I would rather free-form individual and group checks until I and my group reach a fictional conclusion that is satisfactory to all of us.
The PCs are supposed to engage with the fiction and provide their own ways of overcoming the challenges you present them leaning on their skills and abilities. I don't see how your example is any different.