Signal would have to create a fake user for every integrated Whatsapp/Telegram user, and the client would have to create an e2e key pair. Signal's zero knowledge policy doesn't need to be compromised in order to achieve that. Hopefully it would be possible to opt out of the integrations.
This was why XMPP failed. The UX was terrible because different clients didn't support the same features broadly and it was a big mess. And everyone just went with centralized services where everything works as expected.
