This is what is anti-trust law is for.
The problem is we are getting walled gardens of 2-4 companies in many domains (voice assistant, mobile phones, cloud providers, IOT platforms, App Stores, consoles). Each company can be framed as both a double digit participant in the segment and a monopoly on their own platform. In each of these there is competition, however there is often little incentive to take a different path, and instead adopt a duopoly position.
I really think they need much broader definitions of anti-trust, including use of a non-dominate platform to unfairly compete in a different area.
There is nothing wrong with definitions of anti-trust, it’s the enforcement that has been gutted since the 1980’s.
If there had been reasonable enforcement GAFA and many others would literally not exist today to begin with. There would be no regulatory approval for all the merging.