ON a similar note, I still see that AIs give up too soon. Is there a way we can toggle the conditions somewhat? Sometimes I just don't get why an AI just surrenders when they still have a large available fleet and several active planets....
(edit): sorry for the wall of text. That ended up way longer than I thought it would when I started hammering away at my keyboard
I did a lot of experimentation into the AI's surrender behavior years ago, and while I haven't repeated my tests in the latest version of Rebellion I have no reason to believe it's categorically changed. The reason I did those tests (besides my own curiosity) was because my experience was very different from the conventional wisdom: I find that the AI waits much too long to surrender and should have thrown in the towel considerably earlier.
In my tests I found that there are two factors that dominate the AI's surrender behavior: the size of your fleet, and the size of its fleet. Everything else doesn't seem to matter, or if it does the level of weighting is too low to be noticeable. Note that fleet size is determined by the amount of command used; the quality of those units, or even whether they are under construction does not factor in.
If you can keep your fleet size small enough, the AI will literally never surrender. As one of my tests I boxed in an AI on a dead asteroid while keeping my fleet very small and eliminated all of its units and structures while camping on that asteroid. It refused to surrender, and activities like colonizing more planets, increasing my income, researching technologies, and building starbases seemed to have no effect. I then started building units and the moment I queued up enough it surrendered. I've also seen this the other way around, particularly with cruel and vicious AI's, where the AI has a huge number of units queued up at its factories but since I'm killing them faster than it can build them the backlog of queued units just grows bigger and bigger. The AI will refuse to surrender in these cases (since its command usage is very large) but the moment you blast its factories or the colony hosting them it surrenders on the spot because it didn't actually have that many units.
Notably this behavior is irrespective of the size of the scenario, and seems to give more weight to absolute rather than relative difference in fleet size. Smaller scenarios with faster completion times tend to have smaller fleets, and with such small fleets the AI is incredibly stubborn and refuses to surrender well past the point at which it would have reasonable to do so. Meanwhile, on large multistar scenarios it will often throw in the towel just because it can't afford the size of fleet it wants. The AI's difficulty also plays a huge role; lower difficulty AI's are much more likely to fall into an economic death spiral due to upkeep and be unable to maintain the fleet size they want. Higher difficulty AI's, vicious particularly, seldom have this issue and are much less likely to surrender before they are beaten down to their last factories.
However, I took one more step in my analysis: I provoked dozens of AI surrenders in all kinds of different scenarios. Different difficulties, scenario sizes, game lengths, player counts, and strategies on my part. I also reloaded several of these to compare how different behavior on my part affected its surrender (fleet size was the only thing I found to matter). I checked the replays and looked at the AI's current position and evaluated whether surrender was reasonable. In every case, I found its position was unsalvageable. In fact, in almost every case it should have made its last stand long ago and it was well past the point of having any reasonable chance of turning around its losing position.
My verdict then was that the AI's surrender behavior was, if anything, a bit too conservative. The bigger issue was that the AI starts acting very timid when it's outgunned. This causes it retreat from its front-lines and simply cede territory to the player. While this isn't necessarily a bad move, the AI is relatively inflexible and has no realistic way of turning this sort of situation around. As it loses planets its economy suffers enormously (since the AI gets most of its income from tax and extractors) and as it retreats it often loses units to bombers, meanwhile it's constantly blowing cash on new starbases and replacing units. When the AI finally does surrender, it feels arbitrary. The AI doesn't die with a bang but a whimper because it just runs and runs until it finally surrenders because of how outgunned it is, when it really should have made its last stand long ago.
Overall, I can understand why the developers wouldn't do anything about this. AI's are complicated and there's every chance they could make things worse. Short of an option to remove surrender behavior altogether I don't see any reasonable course of action for them at this point. However, I do find it to be quite an interesting topic.