I strongly second this comment. I have been utterly horrified the few times in my life when I have come across arguments along the lines of "personal identity isn't a coherent concept, so there's no reason to care about individual people." You are absolutely right that it is easy to steel-man the concept of personal identity so that it is perfectly coherent, and that rejecting personal identity is not a valid argument for total utilitarianism (or any ethical system, really).

In my opinion the OP is a good piece of scientific analysis. But I don't believe it has any major moral implications, except maybe "don't angst about the Ship of Theseus problem." The concept of personal identity (after it has been sufficiently steel-manned) is one of the wonderful gifts we give to tomorrow, and any ethical system that rejects has lost its way.