Why don’t we also take the fetus’s future into account?

Who said we don’t take it into account? The prohibition of murder is determined by the present. The rule “desecrate one Sabbath for him so that he may keep many Sabbaths” speaks about future values (and indeed is applied to desecrating the Sabbath in order to save a fetus)

So in the example I gave about the dead person who will come back to life, in your opinion would it be permitted to cut off his head? (He won’t come back to life if he has no head.)

It seems there is no prohibition of murder here. It’s like someone who does not impregnate his wife and thereby prevents the birth of someone.
This should be connected to the well-known inquiry of the Chacham Tzvi about Elijah the Prophet’s wife, and I seem to recall that Kovetz Shiurim discusses whether in the future to come we will need to betroth our wives again. But even according to the side that says not, it seems to me that with regard to murder there is no murder here.

When you said above, “the prohibition of murder is determined by the present,” did you mean the halakhic prohibition of murder? Or the moral one? Or both?

First of all the halakhic one, but in my opinion the moral one too. But in the realm of morality, the question of whether this is a prohibition of murder or some other prohibition has no significance, and it is clear that both are forbidden.

So from a moral standpoint, does the problematic nature of abortion stem both from the fetus’s present and from its future?

Isn’t that obvious?