I understand that. The thing is, this entire ancient, complex system doesn't care either way; it could grow to encompass the Earth, or transform into something even more complex or entirely different, or disappear tomorrow - whether due to a volcano, an asteroid, or a bunch of construction workers with backhoes, paid to flatten it and pave it over for $reasons.
Point being, from all of the universe we've seen so far, we only know of one life form that cares. This is us, humans. So unless we learn of other life that has the capacity for conscious thought and care, preserving complexity of nature at the cost of humanity is the extreme case of throwing out the baby with the bathwater. All of that is meaningless without us around, because it's us who process the meaning.
Disneyland with no children, Moloch, etc.
EDIT: I realized that human-centric perspective may not sound convincing in context, so how about:
Naturalism is basically a religion that never grew a holy book. All the complexity of nature, ecosystem of balance, etc. is sure beautiful, but it cannot have primacy or intrinsic importance, simply because it's fake. It's an artifact of our perception, that's limited to a specific time scale. On our time scale, rainforests and coral reefs and all the species most beautiful are stable enough we can identify and name them. But it's an illusion - it's all dynamic and constantly changing; even with no human input, the things we find beautiful today will likely not exist in ten thousand years. A glacier melts, rocks block a river, half a continent of rainforest turns to swamp or desert, everything dies - while reverse happens elsewhere. Etc.
Wishing to sacrifice humanity over an illusion that exists only in naive minds is not a particularly great idea.