Doesn't that seem like a bias? If what we care about is average income, then we should talk about average income (and about more than just the most likely outcome if necessary), not switch to talking about median income just because it's easier to make predictions about it.

Also, I think your "nature is doomed" depends on average, not median, income to approach subsistence level. Suppose you have one individual who has half of the world's income, and the rest are living at subsistence. Clearly that one individual can preserve much of nature by him or herself.