We have best practices and movies about having too little control of other people's systems too. If law, morality, and debate only required finding whether there was a negative outcome they would be trivial. Unfortunately that doesn't yield practical results. Things need to be weighed and debated on a larger scale.

Throwing in my perspective: I largely agree with the DMA and think while iMessage was found not to be popular enough to qualify in Europe we should have something similar in place in the US and it certainly has enough penetration here to qualify under the same wording. I.e. I think we need something between "free rein" and "monopoly" for very large players which has practical effects on how you're required to interop.