It will get rid of 20 massive power centers that move politics for their own benefit. Turning those 20 companies into 500 companies would allow for political change that is impossible now.
No it wouldn't. You are understimating the number of westerners who quite prefer their standard of living and either don't care or have theological beliefs that allow them to not care about the consequences of their lifestyles. There is a cultural change that has to preceed the political one
It would be great if you could fix politics by breaking up a few villainous corporations but it's much more difficult than that. In America, Michael Bloomberg and Donald Trump have (in different ways) shown us the limits of the power of money in politics.
I'm pro-anti-trust and I believe that breaking up certain big corporations would a good thing. But that's a long way from "the problem is capitalism and imperialism" and the implication that eliminating those things is possible or would fix environmental problems. The Soviets were even worse for the environment.