Is Chrome the default browser on any major desktop platform (Mac, PC or major Linux distros)?

If it's still not: Users must actively take the step of downloading and installing it. Is this step less difficult than changing the default search engine in Firefox or Safari?

It is the default on android which is the most used OS in the world. And part of the reason why people install chrome is the heavy advertisement and annoyances that Google put on using it services on something other than chrome. That's the textbook case of monopolistic actions.

And Google guide people to how to install chrome and majority of people can download some exe and run it with next next while keeping the defaults. And many of them will not change any of the settings. And chrome will take every chance to let you revert back to google search if you happen to change it.

Literally nothing qualifies, that's the crux. Nothing is monopolization as defined by legislation because congress didn't do the work to define anything.

The entire antitrust regime from it's inception, all of the caselaw, is a mixture of judicial activism, chest thumping, and executive branch blackmail. It should all be struct down. Start again.