...The human body is constantly being replaced like the ship of theseus so is it still considered alive and not died inbetween the change of cells? What if teleporation is real? You maintain the same Structure. or make a clone of a person? Or you are replaced by electronic parts that have the exact same function?
Even the functioning form that the parts constitute gradually changes and deviates over time, as well as the psychological patterns. One at the age of 40 is hardly who one was at the age of 5. Since both minor and major memory loss is possible, as well as alterations creeping in from its duplication to shifting storage areas, these incremental deaths of developing stages seem to need more than just memory to integrate them as the same individual (or an appeal to such).
Time as presentism, where only a fully distinct and objective "now" is real, as the whole universe is annihilated and born anew in a sequence of replaced moments, chains everything to a similar problem. Skepticism can be engendered because the evidence that each replacement state of the cosmos is still somewhat similar to the one that preceded it is grounded in judgements dependent upon personal and environmental memory (a circularity of memory verifying its own reliability to not only itself but for the entire situation).
As recognized long ago (not by the uninterested majority of commonsense but those with a passion for asking if reality truly does hang together well), some manner of eternalism is needed to anchor existence in general -- not just the supposed persistence of the same human particular. That is, the reality offered by changing experience, when taken either as is or reified to "aphenomenal" physical entities, tumbles into being suspect as a wildly malleable slash inconsistent dream or process floating on its own (with emphasis on "wildly malleable / inconsistent").
This eternalism solution need not be a block-universe or related. The ancients also offered an intelligible world, the forms of which today could be corrupted into any number of hybrids and chimeras (and already have been).
Berkeley reduced people to immortal minds participating in a mutual world distributed by a God version of The Matrix.
Leibniz rescued unconscious rocks, trees, and the like by correlating such appearances to primitive monads too "stupid" to discern their pre-installed, unfolding representation of the universe.
Kant discarded what he considered Leibniz's speculative / unwarranted panpsychism of even the lowliest monad containing an internal representation, by introducing the unknowable "thing in itself".
Back in the '70s, some hippie physicist of Fred Allen Wolf's clique might have interbred the quantum wave function of objects with the intelligible form, producing some concoction that eternally contained all the possible states of a particular human body, house, moon, etc. (including the universe).
The latter ventures just being a round-about way of pointing out that maybe a block-universe or some newer model of spacetime wasn't so bad in comparison.