If one seriously considers the future self as a separate self, it seems to me a serious challenge the Szaszian idea that mental illness is just extreme preferences and that suicide should be respected and allowed as a legitimate exercise of choice; if our future selves are separate selves, then suicide is murder. [Emphasis mine.]
TGGP disagrees on the ground that our present selves and future selves have such united interests that they should be thought of as a single entity. Practically speaking, "If suicide is murder, then spending in the present is theft from a future self, sex is rape and a boxing match is battery," says TGGP.
This is certainly the reason that "successive selves" thinking will never catch on, true as it may be. Then we couldn’t lock people up for rapes and murders for long periods of time. (How do you punish a past self?) The entire justification for contract enforcement is destroyed.
But I think there's a deeper reason that the suicide/murder analogy fails. I respond:
My future self is not anything other than a possibility. It’s a possible self. Even accepting the successive-selves view, suicide is no more murder than is abortion or contraception.
There’s a distinction between protecting the "right" of merely possible people to come into existence on the one hand, and protecting the interests of future people provided they come into existence on the other (as we do when we consider, e.g., environmental protection, budget deficits, etc.).