Not a lot of competition? You have Snapchat, FB Messenger, Whats App, Imessage, RCS (mainly Google's protocol at the moment), Instagram, Signal, Telegram, Discord, Microsoft Teams, and the list goes on and on with messaging apps. To you comment about the just messaging a part of the rules and that is what I was getting at.. currently the difference in messaging apps are the UI, Message specific capabilities, and who you can talk to since currently the only way to talk to another person is using the same platform.There really isn't a lot of competition with messaging platforms in the first place after a decade of network effects entrenching the market leaders. You can more or less ditch one for the other without really noticing a difference other than all your contacts not being there.
If I recall correctly the DMA doesn't require interoperability of all features, just end-to-end encrypted messaging and sharing of certain media files. I think, coincidentally, this is also what locks most people in, so in a sense this could increase competition on features that would truly set the services apart.
I do think that there's a lot of good technical reasons why interoperability might be a tricky thing that's not easy to do and we'll see how this plays out, but I don't think there's a strong argument to be made that it will make the services blander. Quite the opposite I'd assume, even if it's just a better interface.
Yes interoperability will be difficult because not all messaging platforms are used necessarily for the same reason. With that aside going back to what I originally said.. for one.. what do you draw the line for messaging platforms? because things like discord could get really tricky between individual chats, group chats, voice calls, that may be happening simultaneously. With that in mind Snapchat is also another specific messaging platform just because of the way that it is used. Some messaging platforms are popular because of what they did that set them apart, of course others copied, but what this part boils down to is what is the point of me being able to send a snapchat to a friend that can open in say whats app, if I also want to see stories/reels/whatever and I still need the individual apps and how will conversation management happen when opening these various apps? Are we going to open an app and wait until all previous conversations from other apps to load?