I wouldn't be. I canned my subscription to it. If Apple TV+ opened up everything in the iTunes Lib then yes I would sign up right now. But since its not...
That'd be nice but my guess is negotiating the rights and revenue split would be the roadblock. If they can show they and the rights holders will make more money forgoing rentals by putting it on ATV+ then it could happen. Even if they did put it all on ATV+ they could still sell copies as well.