I typically play a style where I am an aggressor later in the game. So I don't usually have incredibly stacked units other than a few and my scouts.
I'm also a big fan of turtle strategies, and it's the main reason why Assyria is my favorite civ, but when playing other civs and staying peaceful for a long time, going into an industrial era war without tier 4 promotions is a big struggle. Once I get rolling, though, I can really start making headway into the enemy territory.
disagree ive seen the AI steamroll myself at even tech: authority AI will beat an even teched peaceful human due to its bonuses and equally peaceful AI, ive experienced defeat to a much stronger AI (1 in 10 ganes) but its mainly an even keel due to AI tactics
I haven't gotten utterly overwhelmed
in a while (maybe it's time for another attempt at bumping my difficulty), but recalling the times it happened with even-tech civs: whole world declared on me as very wide Polynesia; I started between Assyria and the Aztecs as Egypt; and old-patch Genghis Khan ate the entire world and hit my 2-city coast with 30+ ships
Joint wars are what really facilitate getting run over IMO. Even in the worst position of fighting Denmark, Iroquois, or Songhai in their favored terrain, I'm at the point where I can anticipate and fortify my borders enough to lessen the inevitable damage if I fight them alone, and after their power spike, I might even be able to get a city off them.
It's also been mentioned in this thread and in the congress, but indirect fire does feel like a big contributor to the absolute power of tier 4 promotions. It can turn chokepoints into meat grinders, and make heavily forested terrain a deathtrap for the AI. Going from Cannons to Field Guns brings a big shift in strategy largely because of the indirect fire. On archer units, I almost always pick it before range because of the versatility. I know I mentioned more effective defensive options before, but even still I almost never choose cover on archers when I could have this combo; being able to shoot an enemy that cannot retaliate is 100% less damage vs. 50% less only against other ranged.
As a slightly off topic (since this is a brainstorm thread ill throw in my hat) if you wanted to simplify and make promos a bit more meaningful we could attempt almost a class system.
I feel that this is essentially what the main post is describing, albeit without the xp cap. There's already a bit of this for units with medic and cover, but forcing even more specialization sounds appealing to me for taming the power of highly promoted units. It would be interesting to see march units who have no hope of also grabbing overrun, mobility, and medic promotions (or maybe only one of those, much later on) due to deeper branches in the tree.