The proposed extrapolation feels like a tautology. Can you better explain what dynamic(s) you are envisioning to exist and have significant impact, that result in confounds requiring complex numbers to explain probable mortality over encounters from level 1 to 20?
One perspective seems to be that what happens in an encounter is adequately explained within the encounter: strength of foes, clutch abilities, player choices. "Adequately" is noted here because accuracy and insights are sought over precision, which isn't available anyway. It means there could well be other things going on, but they aren't significant enough to matter for what we want to know.
Another perspective is that what happens in an encounter can only be adequately explained by looking across encounters: resource depletion, narrative arcs. My experience chimes better with the first perspective hence I raised it. The second perspective seems dissonant to me.
At this point, I am unsure if you are trolling me, since I've explained it so many times already.
Neither of your statements above are correct. COMBINING them is correct.
A lethality of an encounter has to do with BOTH (a) the foes you fight, terrain, hazards, and all the rest that are inherent to the encounter, and (b) the resources you have for the encounter.
Let me try another example. Picture that a primary damage dealers for one group is a paladin and a full caster doing direct damage, going against a series of tough-but-survivable encounters.
Case A: They know they only have one to two encounters per day. The paladin can Divine Smite on every hit. The caster can use their highest level slots and not worry about it.
Case B: They don't know how many encounters they will have before they can long rest, say 4-6 on average but occasionally going out of it. At this point they are more the conservative in using resources. The caster might pull out more cantrips to preserve spell slots, and will definitely be using lower level slots, based on their judgement of the toughness of a particular encounter. The paladin either runs out of slots to smite or is more careful about using them (maybe only on crits and vs. glass cannons), which either reduces her damage for all encounters or keeps it high for a few and then much lower. The same encounters, because the party does less damage, leaves the foes with more actions and more chance to kill. Chance of a death goes up.
Case C: The group from Case A is happily going along, and suddenly the DM unexpectedly throws a single 10 encounter day at them. They have used up all of their resources early on, and encounters that would normally be trivial for them they don't have the resources to bring to play. Both the paladin and the caster are solely using their at-will options - weapon attacks and cantrips. But their damage is a fraction of what it was before (especially no AoE) and they are out of other options (fly, greater invis, Smite spell conditions) that they could use to enhance themselves.
Before we go on, if you don't understand this, reread it. I'll wait. It's showing reduced resources reducing party effectiveness. Even when the reduced resources are "saving some for later".
Really, this is the part you seem to keep asking questions about and it seems to me doubting really exists. If you don't agree with this don't bother with the rest of the post, it builds on it.
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Now, if you run the same 10 encounters past the same group, you get:
Case A: Split across 6-7 long rests. Lowest chance of character death.
Case B: Split across 2-3 long rests. Not-as-low chance of character death (but still low, it's hard in 5e to kill characters)
Case C: No rests from the start. Highest chance of character death.
If you put them against an overwhelming encounter with a high chance of death:
Case A, B & C: All three can die - it's not just resources left.
On the other hand, if the last of those 10 encounters is quite hard, with a decent possibility of a character death, you get:
Case A: The party goes in at full efficiency and has a decent chance of character death.
Case B: The party goes in with reduced efficiency but still has been keeping some resources in reserve. They have a greater chance of character death than in A.
Case C: The party goes in with a greatly reduced efficiency since they blew through everything early. They have the greatest chance of character death.