There's no particular reason preventing us from having interoperable E2E; we could already have interoperable E2E e-mail today if we wanted. Securely bootstrapping key exchange could be done through having one phone take a picture of a QR code on the other phone. Though, to be fair, I have no idea if any existing E2E services actually let you do this. "In-band"/electronic key introduction does require the message server operator to act as a trusted bootstrapper, but nobody seems to be worrying about that as-is.
The real concern with interoperable messaging is antispam. E-mail was an absolute disaster because there was no barrier to entry for someone who just wants to send unsolicited garbage to everyone. Google killed federated messaging for Google Chat back in the day because for every one person running their own XMPP server there were hundreds who realized Google was just giving away valuable real estate on everyone's Gmail inbox to this chat service. The EU appears to be trying to mandate federation to fix the competition problems involved with iMessage[0] and I genuinely hope there's a user opt-out for this when it inevitably gets abused for spam.
[0] Which, ironically, is more of an American problem than a European one
> E-mail was an absolute disaster because there was no barrier to entry for someone who just wants to send unsolicited garbage to everyone.
Email spam is possible because of the expectation that you can contact someone you've had no prior interaction with. (Also, it at least used to be quite easy to spoof the sender address because participants couldn't cryptographically prove their identities, but we now rely on tech like DKIM to mitigate this).
Messaging apps have an easier job because the services can refuse to deliver messages to people who haven't already received your public key (or some short, per-contact, pre-shared secret).
In fact, email encryption could also work this way, if users first had to send a standardised introduction message, and servers rejected any further messages until the recipient had marked the sender as trusted.
The real concern with interoperable messaging is antispam. E-mail was an absolute disaster because there was no barrier to entry for someone who just wants to send unsolicited garbage to everyone. Google killed federated messaging for Google Chat back in the day because for every one person running their own XMPP server there were hundreds who realized Google was just giving away valuable real estate on everyone's Gmail inbox to this chat service. The EU appears to be trying to mandate federation to fix the competition problems involved with iMessage[0] and I genuinely hope there's a user opt-out for this when it inevitably gets abused for spam.
[0] Which, ironically, is more of an American problem than a European one