I have been playing more Civilization 6, testing out the various win conditions. I have mixed feelings on this issue because while I like a lot of the innovations and subgames of the various win conditions I really don't like the balance of the game in terms of how you go about winning via the various conditions.
The fact that the religious game requires you to balance fighting with regular units of soldiers and fighting with religious units is kind of neat. There isn't much variety in religious units but I liked the decisions I had to make about which ones to make based on what I was trying to do and layering religious combat over the rest of the game felt good to me.
Also the fact that your religious units fight by throwing lightning at each other never gets old. Boom! Crack! Kapew!
I also enjoyed immersing myself in the culture game. I built tons of archaeological museums and sent my archaeologists all over the world fetching things. I made art museums and directed Mark Twain and Jane Austen to various cities to write their novels and impress visitors. I enjoyed figuring out how to theme my museums and chasing particular kinds of great works to fill out the museums I had. It was an enjoyable subgame to the rest of the Civilization game.
But in both cases I was just kidding myself. I conquered an early opponent or two, then just sat there building my stuff for the rest of the game. I built all this culture stuff but it would have been far easier just to keep on with the conquest, smashing my enemies beneath my feet. I could have just removed every opponent except one, and left that one opponent with a single city that had every tile pillaged. Then I could win a cultural or religious victory quickly and effortlessly.
By far the best way to win one of these other win conditions is to simply kill every opponent but carefully avoid taking their last capital so you don't trigger the domination victory condition, and then do whatever you want. The AI is bad at war so this is practical.
Civ 5 had this same issue in a lot of ways. Players could keep their units alive forever by retreating and healing and using ranged units effectively. If the player couldn't heal their units, or if they couldn't pay gold to upgrade obsolete units to new ones, this wouldn't be much of an issue. Warring players would need to build new units constantly to replace old ones and that would be a huge drain on their other endeavours. But as it is you can just build one army and keep using it throughout the entire game, with only occasional replacements needed for units that die.
If replacing units was the norm then there would be much more meaningful tradeoffs between conquering and building, which I would really like.
At the moment I am thinking about ways to implement this in Civ 6, which apparently means I am staring directly at the rabbit hole. I spent most of a year modding Civ 5 and while I enjoyed it greatly it really took over my life. I know that my alterations would not improve the game for everyone, but from reading forums I have found that most people agree with me that conquest is far too easy.
There are other things that make conquest too simple in addition to unit healing. Upgrading armies slowly over time is important, but instant upgrades are also a problem. When your civ researches Crossbows and then instantly your entire army becomes Crossbowmen that is a problem. The instant power increase is immense. So both the fact that you don't have to make new units and that there are enormous power spikes is an issue in my mind. The other thing is simply that ranged units are too good.
Slingers have a range of 1. They are mediocre - good at defending cities and encampments, fragile in combat. When Slingers upgrade to archers though they acquire a range of 2 and are absurd. You can hide them behind melee units, tear down walls, and a clump of them will annihilate any melee unit that gets too close. Crossbows also have a range of 2 and are equally brutal, as are Field Cannons. But when you upgrade them again to Machine Guns they suddenly drop down to having a range of 1 again and then they suddenly aren't particularly powerful anymore. Strangely the attack values of ranged units and bombard units all proceed in a predictable, linear fashion, but the range of the units appears to not have been accounted for in their cost or overall effectiveness.
Archers, Crossbows, and Field Cannons are fantastic units, far too good against the AI. It just can't figure out how to attack entrenched ranged units and when you use a bunch of them you can just slowly march forward, massacring their units, and then blow up their city defenses when you get there. If these units had a range of 1 the game would be a lot harder on the human player because their primary way of killing enemies without incurring losses would vanish.
Implementing these changes is widely variable in difficulty. I don't know that you could remove healing without wrecking the game - the AI in Civ 5 certainly couldn't handle it and kept trying to heal anyway. Destroying unit upgrades is a lot more feasible, and I suspect the AI could handle it just fine. Nerfing the range of Archers, Crossbows, and Field Cannons I know the AI could deal with. One other advantage to the nerfing of ranged units is that bombard type units would suddenly have a reason to exist. If it is really hard to tear down cities with ranged units (because they have to walk up right next to the city and risk being beaten up) then having a catapult or two that have a ranged 2 attack to bombard the city walls suddenly sounds a lot more appealing. I like the idea of rewarding intelligently created mixed armies instead of just 'spam the best unit' as a guiding principle.
Now I need to sit back and decide if I am going to take the plunge and dedicate a couple thousand hours to making this next version of Civ into the game that I most want to play.