I think people are ignoring looking at this through the lens of anti-competitive behavior. Right now there is an alternative, yes. But Apple continues to grab the marketshare in the US (and some Asian and EU markets). However, there is no guarantee that will be forever. Sure, they support SMS now, but again, no guarantee that'll continue to last.

Apple has displayed on numerous occasions that they do not care about interoperability with other platforms and have even been outright hostile and aggressive against them. Just look what happened when some kid figured out how to make iMessage work on any other platform. Sure, that kid's solution was hacky, but he was 16 years old. If one kid can do it, then there's absolutely no justifiable reason seasoned software engineers can't figure out a secure solution.

It astounds me that there are so many people defending any company that not only encourages walled gardens, but in some cases aggressively enforces it. Yeah there are alternatives, but people are lazy and seek convenience. iMessage just works by default, and so many folks get annoyed or even sometimes confused when non-Apple users ask them to use a 3rd party app to communicate with modern features instead of being stuck with SMS's severe shortcomings.

That's why I think the DOJ is justified in this. Because it is anti-competitive behavior.