An unknown, big change is just as likely to be good for humans as it is to be bad.
That is massively untrue. An unknown, big change is much more likely to be bad for humans. When you're dealing with a hugely interconnected web of an ecosystem, big unknown changes are likely to alter the system, such that it stabilizes in a different state. The current state of the world is very nice for humans. An end to many easily caught food species will not be so nice.