Not sure whether you would call this technical, but the difficulty lies in allowing third party access and still prevent spam.
The reason Whatsapp won out over competing services in the first place (over here at least) was that they managed to be both free and relatively spam free. All free alternatives quickly got subsumed by spam (even non-free SMS has a spam problem nowadays).
I guess if you count "silently blackholed by the other server with no recourse" an acceptable result then Apple / Meta can offer you that kind of interop too.
Here's EU telling Microsoft how to conduct business globally, back in 2004 - https://economictimes.indiatimes.com/news/international/ms-s... - 'help rivals connect their products to the Windows operating system.' does not mean 'EU rivals', but any 'rivals', outside of the EU as well.
'Intel v Commission (C-413/14 P, 2017)' is another case where EU Antitrust explicitly punished global conduct outside of the EU.
Right now, with exception of antitrust, EU laws only incidentally affect global conduct, e.g. once a business is compliant with GDPR, it's often too costly to restrict compliance just to the EU.
Nothing stops that from changing. EU absolutely can make a law that obliges e.g. chat app providers to either apply EU privacy standards globally or face bans/fines/seizure of their EU operations.
A probable implementation is that you bootstrap the initial key exchange using web PKI (if you want to talk to Alice@example.com then your client makes a TLS connection to example.com and asks for Alice's public key) and thereafter you use something like the Signal ratchet thing.
It's not a technical problem. It's a political one