Don't fall into the fallacy that "things popular with young people" means that they will stay popular as they age. The social pressures that are driving people to use iOS (iMessage being the preferred messaging app) falls off over time.

notably this is US only

True, but it’s a valuable market, and Apple is most easily regulated from its own country if the U.S. decides the monopoly is causing market issues.

Also, i consider that the US is strategically interested to maintain the semi-monopolies of Google / MS / Facebook because of their worldwide influence. But apple being a predominantly american monopoly, is more likely to draw antitrust litigation.