I think this sidesteps the underlying intuitions too quickly. We have cognitive mechanisms to predict "our next experience," memories of this algorithm working well, and preferences in terms of "our next experience." If we become convinced by the data that this model of a unique thread of experience is false, we then have problems in translating preferences defined in terms of that false model. We don't start with total utilitarian-like preferences over the fates of our future copies (i.e. most aren't eager to lower their standard of living by a lot so as to be copied many times (with the copies also having low standards of living)), and one needs to explain why to translate our naive intuitions into the additive framework (rather than something more like averaging).