Aren't the scenarios I hinted at in my post 3 years ago still an issue?
https://forums.macrumors.com/thread...conversations-in-ios-8.1762454/#post-19462008
For example, what if Joe has an iPhone which receives iMessages to both his number and his email, and an iPad which only receives iMessages to his email. He gives his number to friends for private iMessage conversations and leaves his iPad at home with the family.
You and Joe have a private iMessage conversation using each other's numbers. None of this syncs to his iPad, because that's how he wants it.
Joe is sitting on the sofa one day with only his iPad to hand. He iMessages you, and it gets sent from his email. You want this to go into your existing conversation with his phone. On the face of it, it might seem ok. But where do your replies go? You now have a thread which has iMessages from multiple sources. Where do your replies go? Do they go to the original source, his phone? Then he won't get your reply. Do they go to whatever it was his last message came from - in this case his email? That means you can't continue a private conversation to his iPhone until he resumes sending you iMessages from his iPhone.
I'm sure there are lots of other complicated set ups which make the solution not as simple as you think.
As it stands, I think it's right that each source of iMessage gets its own thread, no?