There’s the Messages app and the iMessage protocol — two different things. How would Apple allow messages from Android that are not SMS? That’s by adding RCS support, which is coming later this year. It still won’t have end to end encryption (like the iMessage protocol does) because Apple isn’t going to support (as of now) the proprietary and closed extensions Google has developed for RCS. Any Android message that comes over the data network will have to have some sort of encryption. Otherwise SMS is just fine, as it is today.