I think you are wrong here. The entire game as it is now has been designed with 1UPT in mind. Yes there is some nuances that are implemented here and can be expanded upon. But if the key point is resources and development time, you are mistaken.
Consider that to use your approach you should first implement a new UPT system and then rebuild the AI including many systems that already work fine in order to just start to implement war strategies. Also this affect the balance of the entire war system, from production costs to unit strengths.
The alternative of just improving and iron the current system is no doubt much less time consuming and much less likely to need debugging, testing, QA and patching. Those will require alone more resources than the necessary to just improve and deepen the AI.
If you are aware of how software development works, you can easily see that changing in this stage of the game a core mechanic would lend to months of developing and insane costs, in addition to the actual AI development.
This is just not a viable solution and even more probably not a solution most players want since it removes the fundamental strategic elements of combat. Yes making it more simple for the AI to handle but also more boring for the player to use.
"improve and deepen the AI", is months of development time for one man if VP is a reference.
Yes, obviously 1(c)UPT won't change for civ6, the base game, as you say it is a core mechanic that most of the players like, but I can say it's not because of time development to rebalance everything, and for that I do speak from experience.
I've 6000+ hours of play with civ5, most of them where with n UPT, and were actual (fun) playtime. I've logged 4000+ hours in civ6, most of them being autoplay to test stability, but the few actual hours of playtime were also done with n UPT.
I my mods, adjusting production cost and units strength were just a few days of development time, not even weeks.
And the vanilla AI could handle it BTW, no need to re-code anything, even with heavy modifications of the combat system, I'm not sure that VP tactical AI specialized on 1UPT would perform much better than the vanilla AI in my civ5 WWII total conversion with 2UPT, ranged units attacks limited to adjacent tiles, counter-fire and offensive/defensive support fire.
I did made some ajustement to the tactical AI for that mod, but it wasn't related to nUPT or the new combat rules (which were helping the AI by simply existing) just some hardcoding of fighter's (rebase, intercept, air domination) and destroyer (hunting subs) behavior.
Ho, and go and try the Stalingrad scenario, and come back to tell me how boring/easy it was, please

Balance in design, again, making something that can both please the human players and not penalize the AI too much. They should be important but not too much. They should give bonuses in producing/moving/healing units requiring them, but not completely prevent their construction if you stick to the few deposits per map.This is a key point. I don't think the game has to be substantially changed here. Just tweaking trade, resource distribution and resource requirements will help a lot no doubt. But if resources are too easy to get, the core game mechanic would be lost and I think it is one of the best ideas in the game and should not be simplified too much. In my opinion the path to follow is to implement AI strategies to judge the resources it needs and effective ways to pursue them.
Stockpiling is a good idea (can't say otherwise !), IMO they could expand on it a bit more, then it would be easier to "implement AI strategies".
I understand your point, mine is that I want an AI that can build an empire along mine, not one that act as a human player and will focus on a victory type and rush it, as it should with the current victory conditions. To rephrase, I'd like the victory conditions to be more balanced, and so I'd want an AI capable of building a balanced empire and survive the full game's length.Maybe i expressed myself badly. When i said "game state" i was merely expressing the ability of the AI to judge the situation of the game. IE, how is my map, how are other civs doing and what opportunities do i have according to my current situation. The AI already does this. And this is pretty much the approach taken by the Real Strategy mod. In that mod the AI commits to one or other strategy depending on civ and leader preference, but also on the current situation of the civ and its known rivals in that victory path. Also commits longer to a strategy instead of flipping constantly and takes into account only the available victory conditions of the game. This is building on the AI systems that the game already has. This mod has shown how good this approach is in making a AI more competitive and FXS should take note
That said, Real Strategy concept is indeed good, and from what I've understood of it, should be able to play the "build an empire" game when no victory conditions are enabled by judging other players advancement and adjust it priorities based on its neighbors.