Then you would be stuck with it only covering text message bodies (or binary blobs), rather than arbitrary structured content (voip signalling, location share data, reactions, etc). You’d also have to figure out how to map the key management and crypto identity over somehow, which would then be equivalent to forcing (say) WhatsApp to throw away their current key management in favour of an OTR like thing. At which point you might as well go the whole hog and just speak something like Matrix natively.