Startups do not have to worry about such issues until they themselves become incumbents, which they do after the previous incumbents were brought low by antitrust action.

> they themselves become incumbents […] after the previous incumbents were brought low by antitrust action

Haha, you’ve never built a startup have you? Try it and let me know how easy it is to become incumbent. :)

Antitrust solves this

No, it doesn’t.

Besides, a vicious cycle in which antitrust creates monopolies to only take them down later is a nightmare for consumers.

Seems to have worked out okay in tech, because we have the benefit of technological progress. WinTel dominance is moot because smartphone is the name of the game, and MS was unable to execute. The Mac and Linux were also able to catch up in the meantime, even though Windows is still numerically dominant on desktop.