Why haven't all chimps become humans, and why are there no Cro-Magnon people alive today if evolution is true?

Because evolution doesn't work that way.

Mutations gain a toehold and become a dominant specie, while others decline due to changes in the environment, breeding patterns and competition for the same resources as the successful mutations.

We have monkeys today because their ancient lineage found an enduring niche to live in. But if we humans go to Africa and exterminate them they will be gone forever. We have exterminated many species over the last hundred years. When we finally get to the monkeys and lions and cheetahs, your question will be moot. There are no Neanderthals living today either because we as the dominant primate lineage saw to it 20,000 years ago that there was no longer a niche for them to exist in without directly competing with us. None of this proves or disproves whether evolution is true or false.