The problem with that is with things such as children and the elderly. If it's just a free-for-all, pretty much every child would be left behind, through no fault of their own just because they're not better equipped physically than full-grown adults. Because of children not being strong enough to get themselves to the front, their families will lag behind trying to help them, and then most of the families would be boarding last because they're less strong through no fault of their own. The same goes for older people, that can't fight for a spot against a teeming mass of full-strength adults.
As unappealing as "playing God" sounds, you seem to think that that doesn't happen all the time in different situations already. Organ transplant lists already refuse to admit people if they have a history of living their life wrong, no matter how sick they are or how badly they need the organ because they can't be trusted to not just destroy the donation.
While it's very admirable to have faith in the belief that in a life-and-death situation, adults may give their spots for children, I would rather not put the lives of children up to chance. I would rather organize it as "family, man, woman, elder, family, man, woman, elder", than just hoping that people are nice enough not to trample the children and elderly if they're well aware that their lives are on the line.