At that point, I have two questions:

1. Why can't I just plug it into a Mac/Macbook instead?

2. Does this mean the battery/compute pack will be an optional add-on?

Maybe you can plug it in. But then you'd be tethered to the computer.

However, Mac might not have low enough latency / high enough bandwidth for best experience. My thinking is that it will be an ultra-optimized highly integrated device designed to do just AR/VR, and that's also why it can beat all competitors which interface with standard PCs. I have a very high-end VR headset and GPU, and it just isn't good enough. Apple has to make some miracles here.

The logic board in a modern iPhone is very light, perhaps 10-20 grams?

They would need to take all the tracking camera feeds, compress them and send them over a high bandwidth low latency interface, do the compute, then send the result back up to the displays. That all requires a bunch of interface electronics, which may eliminate the savings from moving the CPU to the hip.