Earlier quoted context omitted.
It's legal to have a monopoly. It's legal to have a monopoly and get advantages in the market. It's illegal to have a monopoly and then try and extinguish your competition in the current or any adjacent markets.
Mergers get blocked simply because the resulting company would have too much market share. That is a straight up ban on monopoly. My given example of dell and Intel actually happened and was prosecuted. How does that fit your claims?
Monopolies and oligopolies are inefficient. Whether legal or not they result in in detrimental outcomes.