I have not gotten to the late game yet, but my sense of it is that AI factions face the same problems maintaining their glory that you do, and that turn 100 is much too early to worry about a couple factions ahead of you. You and they will face major hurdles. (From what I read about difficulty levels, this is the main thing that is tougher at higher difficulty levels – there, AI nations will face diminished internal threats.)
Of course, part of this is also a matter of personality. If I took a minor faction, created a secure empire, and finished with the third most legacy at the end of 500 turns, I would feel like I did pretty well by them. But even if you have to see a victory screen to be satisfied, I think there is a lot of water to pass under the bridge.