I'm taking another look at this and find it hard to sum up just how many problems there are with your argument.

I doubt selfishness is as strong an attractor among people who are the smartest and most knowledgeable of their time.

What about the people who are the most powerful of their time? Think about what the psychology of a billionaire must be. You don't accumulate that much wealth just by setting out to serve humanity. You care about offering your customers a service, but you also try to kill the competition, and you cut deals with the already existing powers, especially the state. Most adults are slaves, to an economic function if not literally doing what another person tells them, and then there is a small wealthy class of masters who have the desire and ability to take advantage of this situation.

I started out by opposing "simple selfishness" to your hypothesis that "Buddhahood or something close" is the natural endpoint of human moral development. But there's also group allegiance: my family, my country, my race, but not yours. I look out for my group, it looks out for me, and caring about other groups is a luxury for those who are really well off. Such caring is also likely to be pursued in a form which is advantageous, whether blatantly or subtly, for the group which gets to play benefactor. We will reshape you even while we care for you.

Individual humans sometimes become more selfish, but not consistently reflectively so

How close do you think anyone has ever come to reflective consistency? Anyway, you are reflectively consistent if there's no impulse within you to change your goals. So anyone, whatever their current goals, can achieve reflective consistency by removing whatever impulses for change-of-values they may have.

the only real attractor in mindspace that could be construed as reflectively consistent given the vector humanity seems to be on.

Reflective consistency isn't a matter of consistency with your trajectory so far, it's a matter of consistency when examined according to your normative principles. The trajectory so far did not result from any such thorough and transparent self-scrutiny.

Frankly, if I ask myself, what does the average human want to be, I'd say a benevolent dictator. So yes, the trend of increasing humaneness corresponds to something - increased opportunity to take mercy on other beings. But there's no corresponding diminution of interest in satisfying one's own desires.

Let's see, what else can I disagree with? I don't really know what your concept of Buddhahood is, but it sounds a bit like nonattachment for the sake of pleasure. I'll take what pleasures I can, and I'll avoid the pain of losing them by not being attached to them. But that's aestheticism or rational hedonism. My understanding of Buddhahood is somewhat harsher (to a pleasure-seeking sensibility), because it seeks to avoid pleasure as well as pain, the goal after all being extinction, removal from the cycle of life. But that was never a successful mass philosophy, so you got more superstitious forms of Buddhism in which there's a happy pure-land afterlife and so on.

I also have to note that an AI does not have to be a person, so it's questionable what implications trends in human values have for AI. What people want themselves to be and what they would want a non-person AI to be are different topics.