That EU law doesn‘t ensure the big players communicate with eachother, it ensures that big players have to provide a way for other, lower userbase apps to tie into their chat network. Probably means that your tiny chat app XYZ hooks into iMessage, whereas the other big players like WhatsApp are in no way forced to actually hook into other big player platforms because they can stand on their own and profit from platform lock-in.Oh really? What about eMail, Html, Postscript, Pdf, Json, Xml?
Apple doesn't vote for better - it just votes for proprietary. But I forgot to mention, that there is already a law being created that addresses exactly this scenario. So Signal/WhatsApp/Younameit but be able to communicate with each other.
Btw - MS DOS was never a standard, it was just an OS ...
Also, Signal doesn‘t fall under the criteria to open up (userbase too tiny).