Comment by diebeforei485

Again, competition law does not focus on the existence of competitors (true monopolies exist only in high school microeconomics).

If a company has durable (even if not a literal monopoly) market power due to its anticompetitive behavior and exclusionary conduct, that is grounds for taking antitrust action.

It's hard to switch to a different smartphone, due to anticompetitive actions taken by Apple such as their app store monopoly and making it harder to distribute web apps - switching to Android means losing all the apps you've purchased.

It is a lot easier to switch to a different brand of kerosene fuel, so antitrust action against Apple is justified despite them having a lower market share than Standard Oil did.

I'd suggest reading a textbook on US antitrust law and the court decisions that have led us to where we are, because this is a large topic that goes well beyond market share.