> companies have a monopoly in one area that they leverage to extend their monopoly into "the next thing."

This is also illegal in the States for being anti-competitive. e.g. if you have a dominant market position in one market, and leverage that to enter another market and/or gain market share, this tends to be the source of many Sherman cases.

Ex: Company X is a monopoly in market A but wants to enter market B. If I use extracted rents from A to significantly undercut prices below competitive level in market B to gain share, even if it's unprofitable in the short term - THAT's a problem.

Note that startups don't get as much scrutiny when they do this, partly because it's very rare for a startup to have dominant market share.