To your first point: yes, that's the whole idea and it's been the same playbook for any colony from the Americas to Australia. Mars is a much bigger challenge but our technology has advanced enough to make this possible. Let those who would dare risk their lives to achieve something great do so. If you don't want to participate then you don't have to, this isn't a national or international project so you can safely sit it out.

To your second: there are calamities for which a second self-sufficient planet would be necessary. A large asteroid impact would utterly wreck the global industrial base which is highly dependent on international trade and specialization of resource extraction and manufacturing in different regions of the world, having an independent backup on a different planet is one of the few ways to hedge against this. You could argue that every country on earth be fully self-sufficient and then maybe they could survive but that seems like a much bigger task since you'd be building multiple completely independent civilizations instead of just one. Guarding against another global pandemic is another and being out of range of nuclear weapons and their fallout is a third.