Hmm, honestly I don't know enough about WhatsApp and iMessage interoperability so maybe you have a point. One sticking point for me is picture/video quality, is this an issue WhatsApp to iPhone? And I mean directly sending through texts, not sending a link. I'd also be curious how group messages truly work as you note, are they truly as seamless as iMessage to iMessage, and do you get all the other stuff like tapbacks, typing indicators, etc.
Finally, it's not about installing an app, it's about coordinating all your contacts to use one service. I have hundreds of contacts, it would be an absolute nightmare to try to get them all on WhatsApp.