for case to be “antitrust” does not requires to be monopoly as such. it can be a company which is dominant in its market by big margin. Microsoft got involved in multiple antitrust cases, even if there are alternatives (mac, linux), but MS has captured majority of market. So I believe Amazon captured majority of online shop market too, so it can abuse situation in their favor.

It isn't illegal to be a monopoly, it's illegal to abuse your monopoly position. Which is what MS got in trouble for.

Big difference which people often forget.

I wonder when it's going to happen.