That is precisely the point. Now, people try to leave the iPhone, but then they have to get their iPhone friends to use a different app such as WhatsApp, FB Messenger, etc. to communicate with them and send pictures/videos. If what I described was in place, then you could use any messaging app to communicate with iPhone users and be able to exchange pictures/videos. The could keep using iMessage and you could use whatever you wanted on another platform. Or, if you wanted to use something else on the iPhone you could do that too.
I never really understood this logic. I have no problem getting someone to install a message app on their phone if they what to message me, especially family and close friends. And the popular messenger apps make it easy, as they use your number and search your contacts that are also using the messenger app.
If someone refuses to message me unless I'm on iMessage, then they are not important enough to message.
The premise that it's too cumbersome to simply install a message app is ridiculous. I've had people I barely know start to use Whatsapp, and it wasn't a problem for me at all to install Telegram when several people I know requested so.