You keep talking about logistics in Civ6, but I find that the current game design takes away as much logistical decision-making as it adds. Let's first agree on terms: what is logistics? dictionary.com defines it thus:
- the branch of military science and operations dealing with the procurement, supply, and maintenance of equipment, with the movement, evacuation, and hospitalization of personnel, with the provision of facilities and services, and with related matters.
- the planning, implementation, and coordination of the details of a business or other operation.
I hope that we can agree that one facet of logistics in Civ is planning how to execute a military operation, such as taking a city. A second facet is the issue of how to best move units to the front, and this especially means reinforcements. Some parts of this are interesting in Civ6, for example considering good routes of attack and envelopment maneuvers. Unfortunately, the extremely low movement speed makes a mockery of most of these attempts in the current game design, so positioning of different units is often more incidental than a result of good logistics and as a result front line maneuvers are much less interesting than they could be.
Additionally, reinforcement logistics are all but non-existent due to free healing of units and efficient buying/selling of units. Where reinforcements are concerned, Civ4 was certainly a much more interesting game where you constantly were building new units in your cities and had to move them to the front lines (this was potentially non-trivial as single units were very vulnerable and could be preyed upon behind the lines if your strategic positioning was stretched). In Civ6, you simply have units sit around for a while and they are automatically healed at no cost whatsoever. Similarly, buying units near the front is very efficient and also a great way to completely remove this part of logistic decision-making.
Moving your veteran units through your realm is not very engaging to me (and I would hazard it isn't for most players, but perhaps you are an exception and like this kind of slide puzzle thinking enough to spend hours with it every playthrough) and you spend a lot of time on "decisions" that are none because the optimal (or at least a very good) way to do this is obvious with a little experience since there are no meaningful trade-offs involved. In theory the computer could very easily calculate the best movement path but fails to do so because it tries to move all your units through the same route, so you have to baby-sit them. The only interesting part about it is that it takes a lot of time - speaking in game turns here, though it also takes an awful lot of real time - which keeps you from being able to bring your units to bear at multiple fronts.
So all in all, 1upt in Civ6 is, to me, not more logistically interesting than stacks were in Civ4. They provide different takes on logistics, but neither is terribly interesting and at least the Civ4 approach took a lot less real time spent on trivial non-decisions to execute it. More importantly, I dislike how the near absolute removal of the need to produce and move reinforcements warps the rest of the game around it in a very bad way, preventing you from having to make trade-offs between military production and civil development unless you screw things up very badly.
Note that these are very fundamental design weaknesses: I haven't mentioned AI even once so far. It must be mentioned, however, that properly leveraging free healing is a big part of what makes human players trounce the AI so easily. Furthermore, 1upt makes concentration of force much more difficult, and combined with effectively immortal player units, the AI has a much harder time leveraging their production advantages on higher difficulty levels. When designing a combat and logistics system for a game that is largely targeted at the single player market, the cost of creating a competent AI for the system you design should be taken into account from day one, not be tacked on later as an afterthought. I agree that the Civ4 AI was not better than the one in Civ5 or 6, but the system made it easier for it to use its advantages, which at least made it seem more competent.
Can these issues be fixed? Probably. But Firaxis doesn't have a very good track record of doing it. Does 1upt have more "potential" to create engaging warfare than stacks? I think so (games like Panzer General of Battle for Wesnoth are good examples of tactically interesting games), but if you do that the tactical/logistics game is likely to become the primary concern and I don't think it would make the game more fun all things considered. I prefer games that decide on whether they are primarily tactical or primarily strategic games to those that try to be both and make neither part of the game very interesting.