It sounds like a dream, but it could end up a nightmare.
“We must standardize the technology so that everyone works on the same sheet of music.
Lock-In
Messaging platforms are valuable because they have considerable lock-in.

Jayne Harris / Unsplash
The DMA would force platform vendors like Apple and Facebook to make their services work with each other.
They wouldnt have to set up the Facebook-owned app at all.
The problems here are utility and security.

Oleg Magni / Unsplash
Apple, WhatsApp, and Signal all use end-to-end encryption to keep the contents of your messages completely private.
It is impossible for the platform providers to see your messages.
How, then, could encryption survive this interoperability?
By allowing interoperability, you remove the platforms lock-in and make them far less valuable to their owners.
Is It Even Possible?
But it wouldn’t be pretty.
Security-wise, it’s certainlypossible to make encryption interoperable, but it would have to use a common standard.
It’s the security behind the little lock in your web online window’s URL bar.
But that would require a lot of work to implement.
And who wants that, apart from the big tech companies the DMA is supposed to tame?