Attempting to invoke nature as a basis for a definition of ethical is not ethical.
Yes, nature gave one person cancer and not another person, or nature makes a young person on average healthier than an old person.
But what we do with the hand nature has dealt us is on us not nature. Saying "Hey it's nature that made that guy require so much resources, there's nothing we can do about that, they simply cost more and so they should have to pay more, or die more." Is both unethical and a lie, because, there IS something we can do about it.
We are not unthinking animals with no capacity to bend nature to our will, or at least compensate for nature we can't bend.
And the people who cost more among us are not objects with no other factors in their equations than what they cost vs what they produce.
When you say "Nature is the one introducing the unfairness." You actually make the very point you think you are arguing against.
You say "It's Nature's fault", which is true, but then you, with very swiss cheese ethics, select only 2 out of the 3 implications from that to care about. You only talk about how that means it's not your fault or any insurance companies fault, and you don't account for the equally true fact that it's also not the cancer victims fault.
If something is "introduced by Nature" your own words remember, then you yourself are acknowledging that it's no one's fault, and therefor no one's responsibility.
Your inventively kinked ethics interprets that to mean only that therefor it's not your responsibility to do anything about it, completely neglecting the perfectly valid opposite option that if it's no single persons responsibility, then it's everyone's responsibility equally.
It doesn't matter what arguments you manage to muster in defense of the selfish and frankly sociopathic response to the given facts of life and "nature", it's all the damning it needs to be merely that you try, other than in a devil's advocate role.