I think the work of Marx is more valid now than ever, we can see it in front of our eyes. Without putting us down, he did a hell of allot more work on these questions (with Engels of course) than we have - a whole life of work, see: www.marxists.org (and many many others).
If you understand evolution, you can see the change occurring, remembering Marx wanted to dedicate Capital to Charles Darwin. If you've read Marx/Engels books you will see they go a bit further than The Communist Manifesto.
Capitalism is definitely unsustainable, but the new grows out of the old, in fact, is created by the old, just as in evolution - Capitalism creates the environment for a new system to develop whatever you want to call it.
Alternatively we can destroy ourselves which is evolution too.