These anti-monopoly laws always end up as “make restrictions preventing new competitors at some cost the big companies can easily handle”.

Yes, that's why the biggest companies are constantly lobbying to pass antitrust legislation. Because of the extensive history of small upstart competitors being destroyed in antitrust cases.

Always? All anti-monopoly laws, ever?

That’s a hypothesis explored by some authors, like here: https://www.libertarianism.org/publications/essays/big-busin...

Libertarian is a word for people who are too bashful to describe themselves as Feudalists.