This sounds nice, but when you really think about the consequences it really is not. Isn't this going to kill features that only some apps have or offer? What if one day Signal, Telegram or WhatsApp want to implement something "different" that could make messaging better but this policy is making things harder to implement because it would be impossible to make it interoperate with other messaging apps? I don't get w…
I couldn't care less if Apple is 'owned' or not, its nothing special to me compared to all other corporations. Having at least a bit compatible messengers is definitely something positive, its quite a mental gymnastics to immediately try to come up with some theoretical reasons why this shouldn't happen at all (and failing at that). If apple didn't do typical apple thing to create its own standards and share it with nobody, they wouldn't be hammered by regulators 15 years down the line (lightning port, now this, we will see what comes next, plenty of topics to pick up).
Apple knew very well this will eventually come, same for USB, but they made it into operational cost of having some sort of advantage to get more income till its stopped. Now correction happens. You regulate such thing because clearly corporations only care about profits. And markets, as wonderful as they are, always put money on the first place so waiting for them to move corps into more considering-society territory is often a fruitless effort unless you have gigatons of patience.