It looks like in a "pure" ECS architecture we will get rid of GameObject all together, and only use Entity. While it makes sense, all the conveniences Unity has provided come as Component, like Mesh Renderer, Hinge Joint, and the Transform hierarchy tree. It would be very tedious to reimplement them. For example, in the Two-Stick shooter example, they implemented their own collision logic. It's okay for a simple game like Two-Stick shooter, but for larger games it will be a ton of work. Plus, are we really going to reinvent every good thing Unity has had? Is there a way to get the best of both worlds?