If you are talking about vs. AI skirmish Conquest mode games, you can set the AIs to DE’s “AI’“ (as opposed to HD, or CD), remember to pop open the chat window sometime after match starts, select AI Commands, scroll down to “104 - Don’t resign” and click it once.
At least that works now nearly 100% of the time and I have been using it for past few months. One or two times it hasn’t worked, but that is much better than the probably 70% of the time it used to NOT work.
Past discussions on this:
If you are talking about Campaign games, I have no idea, but maybe 104 AI command would work? (Doubtful) But I also wish we could continue those until the very end. It’s one reason why I don’t like playing Campaign missions. You spend a bunch of time building your econ and amassing an army, only to have the rug pulled out from you once all objectives are completed.