Also, I want to point out that the moral issues are nowhere near as clear-cut as you (and Kant) seem to think. Even if you axiomatically assert that people have terminal value, you still need to explain why people have that value, whereas trees (for example) do not. And also clarify the boundaries of that protected class "people". (Does it include fetuses, conceptuses, persons cryonically frozen, HeLa cultures, etc.?)
What if I were to ask the same question about why society should be valued?
Is it possible to answer these questions without once veering into the realm of instrumental values?
If you keep trying to justify values instrumentally, you'll wind up in an infinite regress.
Not necessarily.
Maybe what I would discover instead, if I actually charted out my value structure, that all of the things I value exist in an interlocking network that doesn't ground out in any special real, true, honest-to-goodness, fundamental, basic, not-dependent-on-anything, terminal values.
While I'm not committed to the absence of terminal values, I consider the possibility plausible, and I don't find the "well, there's got to be something at the bottom of the stack!" argument for their presence convincing.
A few years ago, I wrote a little dialogue I imagined between 2 materialists, one of whom was for and one against abortion, centering on the personal identity question. I recently cleaned it up and added a number of references for the biological claims.
You can read it at An Abortion Dialogue.
Early feedback from #lesswrong is that it's a 'nicely enjoyable read' and 'quite good'. I hope everyone likes it, even if it doesn't exactly break new philosophical ground.