Memories don't randomly vanish; there's pretty good evidence that the brain does retain useful content. So if you're arguing that "500-years-in-the-future you would be such a different person that we might as well say now-you is dead in 500 years," I think the burden of proof is on that claim because we don't make it for modern elderly people. It passes neither the common-sense sniff test nor the extrapolation test ("What's special about 500 years out as opposed to 70 years out that breaks continuity?").