That's not my premise at all. My premise is that people who care about security can continue to use iMessage or whatever tool they deem secure enough for them. People who don't care that much and would like to use a different messaging app by default, like how much of the world uses Whatsapp, should be allowed to make that choice. You'll still be able to send your friend an iMessage and have it pop up in the Messages app in their phone, since I'm not seeing anything saying Apple have to open up iMessage, but if you send a text message, the already incredibly insecure communication standard that it is, another user should be able to receive that on a different platform on their phone.
You seem to be conflating setting a default messaging app with everyone getting full, unrestricted access to iMessage and Facetime content. That's not what this is saying. Just like I can't receive Whatsapp messages in the Messages app, I wouldn't receive iMessage messages in Whatsapp. I can see how you made that mistake, but it's a mistake nonetheless.