If Apple in 2020 had allowed third-party text message applications and in 2024 changed their mind because a competitor was eating their lunch, that would be anticompetitive.

It's not anticompetitive to never allow a third-party text message application in the first place.

For SMS? Maybe, but that's just a basic phone feature. An old Nokia also only offers one way to send SMS. It's just part of the phone-bits of the phone. And didn't Signal used to be able to send and receive SMS on iPhone, anyway? Thought it could.

Messaging apps that compete with iMessage are plentiful and Apple doesn't get in their way. You can also disable iMessage with a toggle on the Messages settings screen (it's well "above the fold" for me, fourth entry down, on that screen, it's the first thing they show other than security/permissions stuff for the app)