I don't find lutusp's points to be all that unreasonable. While you passionately assert that they're false or poorly reasoned, I really haven't been able to figure out from your post what you think the exact problem is.
If there's not a single species present today which was also present 1B years ago, it does not seem unreasonable to me to suppose that humans will not be around in five times that timespan in the future. If anything, this should be the default supposition, and it is any claim to the contrary -- that we will be around in 5B years, in a form we could mate with -- which needs extra support.
If there's not a single species present today which was also present 1B years ago, it does not seem unreasonable to me to suppose that humans will not be around in five times that timespan in the future. If anything, this should be the default supposition, and it is any claim to the contrary -- that we will be around in 5B years, in a form we could mate with -- which needs extra support.