I appreciate the explanation, thanks.3 and 4 both had serious divergence from each other and from 1 and 2. Stacking in 3 is not relevant to 4, where being in a stack was a liability due to collateral initiative (you could, if you got whaled on by a stack, get it eliminated over the course of 2 turns by a significantly smaller force with no realistic counter-reaction between them in civ 4). The reason is the change to how siege worked in 4 compared to 3. Civ 2 was similarly very different...losing EVERYTHING when you lose once on defense is a pretty crushing disincentive unto itself and might have been a good model with more modern UI controls.
The frustrating thing for me, reading through threads like this, is that players make wrong assumptions about every model. 1UPT is "slow" in civ 5 and 6, but this is not the fault of 1UPT. It's a serious fault of the design teams for those games, but there is nothing in principle dictating 1 UPT has to be slow, that ranged/mounted has to dominate it to alpha-strat degrees, or that city management has to be an input-laden chore. The civ 6 design team left it that way, but blaming 1UPT for that doesn't give it a fair shake.
Movement rules, # movements per unit class, temp stacking similar to when a new unit is produced in a city with a garrison, production rate, relative strength per era/city, functional cycling, accurate displays, accessible information...these are things that can be incorporated into a 1 UPT model. The support unit/other layer concept with attaches is reasonable too. 1 UPT can be made tactically deep and viable. So can stacking or limited stacking.
If the game doesn't do it, doesn't offer interesting choices frequently, doesn't care about rote inputs, then yes it will feel slow...and the difficulty introduced by hidden rules is the kind of fake difficulty that has no place in this franchise.
And I agree completely on the separation between <problem areas> and the attributed cause people like throwing at it. I want these flaws to be discussed in more depth instead of just arbitrarily deciding that one thing is the root cause of it all and following the logic backwards to justify it. Only then can progress actually be made; irrelevant of Firaxis actually making the ingame changes.
We have a modding community which we're all pretty proud of I think, and ultimately we discuss these things because we want to, and not because they might eventually make it ingame in some form. That'd be ideal, of course! But it isn't the primary goal when someone makes a thread, in my opinion.