Hi, I'd like to talk about a strange feature of the game and how potentially is ruining the late game AI behaviour.

I'm talking about Unit Upgrade cost:

many people are complaining that the upgrade costs get too high the more you progress in the game, and that's maybe true, but to me the worst part of it is that the upgrade cost doesn't consider the current level of the unit you want to upgrade.

In simple words: if I want to upgrade a line infantry to a rifleman the cost is the same as if I want to upgrade a warrior to a rifleman. It doesn't make sense.

This way you waste a LOT of money trying to keep your units updated, to the point that is better to wait and research later technologies and then upgrade all units to their latest vesions.

And this is probably what is causing AI to keep obsolete units in late game. Is not about the AI being dumb, but maybe is about the AI being out of money. Sometimes I found myself struggling for money to upgrade units, and by the industrial era I'm by far the richest and most powerful empire in the game. An AI empire, with 3/4 cities, has no chance to earn enough money for its military at that stage. AI starts to show some upgraded units in the very late game, by recruiting them.

I don't know it for sure, but possibly a little change in the unit upgrade mechanic can make it not only more logical, but also more accessible to players and AI, improving the late game experience (and Humankind desperatly needs to do so) a little bit

the major problem is probably not costs, but access to resources (aside of technologies)

In particular AI has big troubles maintaining trade routes

I upgrade units when I need them or if I have too much cash.

Keeping a warrior instead of a swordsman means if you're having battles with that unit, you are at a massive disadvantage. If you are going through multiple eras without using a unit then... it was a waste of money in the first place. Alot of people would say disband it and use that population in the city.

However, I don't have a problem with costs at all. If you choose 1 merchant culture at any time, you'll usually be making enough money to upgrade units. If you never choose a merchant culture, I could see having a problem with the cost.

Maybe the AI algorithm looses track. I did on my big militarist game recently.