What I probably like most of Alpha Centauri would be related to strategic depth and the technology tree; the possibilities regarding how game may be played. AC basically has four different branching paths on the tech tree, and you can approach things differently every time you play - even just with regards the beginning game there is strategic depth. Sometimes I only play the beginning.
Of course, just being able to do things differently all the time is not important to me by itself, that's just a byproduct of the strategic depth of the game. Merging the factions (affinities) function with the tech tree like in Beyond Earth does not immediately suggest strategy to me. It doesn't suggest anything at all to me other than gimmick.
I do not think the other Civ's have as much strategy. For a Civ game, it would just be whether to take the time to research the dead-end cavalry tech for a temporary boost in military, go republic or monarchy, etc, but it hasn't much depth or importance in my opinion.
On AC I might be good enough to win regardless of circumstances (I doubt the AI can kill me), but regarding the strategic possibilities available on how to approach the game, it is possible that I may still not really know what I am doing. Based the possibilities alone, I could be doing things completely different, that might be more optimal.
For that reason, even if Kyrub hadn't just done an update of the AI, the game would not be dead to me. This game will never be dead for anyone who wants content and depth and not just something "new." If you approach it that way, I am not certain that the content of AC actually runs out. The expansion, which actually offered more "content", I do not play, because I think it harmed the strategy. That being said, I like the real-world parallels of the original's factions more than anything mods or the expansion might offer, even while i consider the factions feature over-rated.
I probobably don't care as much as other people whether a game has "more than one right way of doing things," which is just another way of saying that you want more shiny baubles. Other people like the number of factions and such, but these could just be turned into government choices if not for their attachment to the story. And we all know by now that the Chinese Legalist faction (Hive) is the strongest anyway, from play-throughs and tests. You can play another faction but it is just a handicap. Morgan and University can probably research about the same (you'll get their research after Vassalizing them); the game didn't flesh out the Gaian option enough, and the military factions are probably not the best option in the long run.
So I consider the Faction feature over-rated (in it's strategic depth) other than teaching me Chinese history. Beyond Earth will be expanding on the Gaian option, but I don't care about Beyond Earth, or it's affinities (factions) function which it has bizarrely integrated with the tech tree, unless it offers the depth of Alpha Centauri.