The claim that we have absolute limitations is a very bold one. I don't think it's valid to make it without even presenting some sort of preliminary evidence. Of course, there can never be evidence in the way scientists use the word that anything is "absolute". So basically, you are asking us to swallow that. Of course, we don't want to, but rather actually try to obtain evidence by trying to live that long and trying to make the transhumanistic vision real.The probability distribution of all that you can possibly be and do within nature has always been intrinsic to the laws of nature... recognition of our absolute limitations.
Thus:
If we knew that now, then we might indeed have less of a reason to try. Luckily, we don't.What will you be doing for the next 10^(10^(10^(10^(10^(10^(10^(10^(10^10)))))))) years?
Your thoughts on personal identity are fascinating. I would recommend Harris' exchange with Glannon
Glannon W. Identity, prudential concern, and extended lives. Bioethics. 2002 Jun;16(3):266-83.
Harris J. A response to Walter Glannon. Bioethics. 2002 Jun;16(3):284-91.
Glannon W. Reply to Harris. Bioethics. 2002 Jun;16(3):292-7.
In addition: Parfit, Derek. 1971. "Personal identity," Philosophical Review 80:3-27 (Or why not the book "reasons and persons" in the first place)
And importantly: Whiting J. 1986. "Friends and Future Selves" Philosophical Review 95: 547-580.
What it boils down to is this:
Personal identity cannot be caused by psychological similarity, because the former is transitive and does not admit of degrees, while the latter is intransitive and does admit of degrees. Thus, the classical argument from psychological change against desiring long personal lives fails. This is not entirely satisfactory for the immortalist who believes in personal identity, because the invalidation of psychological similarity leaves him without any measurable grounds of personal identity. This is why immortalists are often Parfitian reductionists or personal-identity agnostics.