I thought partiality was wrong before I thought about the coherency of personal identity. I don't see either argument as giving me any additional reason to oppose those things.

That's not my argument - rather, I simply point out the highly limited usefulness of dividing the space of concerns into "altruistic" versus "self-interested" categories. These are not two different kinds of concerns (at least to a clear-headed reductionist), they are just two different locations, or directions of concern. Without locating the concern in a history and causal trajectory, and just looking at the felt quality of concern, it's not possible to categorize it as "self" or "other".

You said earlier:

I want there to be other people in the future, but this is purely for idealistic and altruistic reasons, not because of any form of self-interest.

That alleged contrast is what I find wanting.

I don't have any objection to taking a 4D view of objects, including people. Whatever works for the task at hand. I also don't reject the concept of personal identity; I just put it in its place.

For instance, I believe that a world where a person lives a good long life is better by far than one where a person dies and is replaced by a new person who experiences the same amount of wellbeing as the dead person would have if they'd lived.

A lot of what is valuable in life requires a long time-horizon of highly integrated memory, intention, and action. Normally (but not by any necessity) those long spans of highly coherent activity occur within a single person. There is more to life than moment-to-moment well-being. So I would agree that your first scenario is better - in almost all cases.