It also sidesteps the issue of people who plan/want to live indefinitely through advances in biotech, cryonics, mind uploading, etc. At the risk of distorting 'rational', if one personally cares at all about the future of humanity rather than just oneself in the present and immediate future it's easy to see why a course of action that helps humanity survive rather than hinders is a better, 'rational' course given that person's beliefs. Insert "moral dilemma" of the same flavor as being asked a preference on dying now, with everyone else living on, or living 5 more years then everyone including you dies.