>If you accept an empirical view of the universe, no matter what you do, you will eventually die.

Perhaps you're overestimating the extent to which billionaires accept that view.

>Therefore, the only rational investment would be in life extension technologies (SENS, advanced prosthesis, etc.).

Or maybe the ultra-rich have done enough research to conclude these sorts of technologies won't be mature in time to help them. So why bother?