There's tons of posts out there explaining "how", but not so much justifying "why" beyond the "oh, it's new, it's cool, just move on".
Consider existing projects, much of them medium to large in size and importance. The investment was already done. There's so much tooling and solid project standards that has evolved though time, just like the community around.
They deliver value to users that don't know what's AngularJS nor Angular, React or whatever.
So, beyond the "it's cool" hype, why should we consider migrate existing AngularJS projects to Angular (or anything else)?