IF it were possible to slash cost that way... it sounds great but there are big technical limitations.
The Vision Pro uses a lot of compute power that a Mac can't replicate, namely:
1. The R1 chip for real time low-latency passthrough, foveation and eye tracking - that needs to be done on a real time chip and couldn't work with something that uses a standard CPU architecture, even less something that is down a wire on a computer with other tasks that compete
2. The neural engines of the M2 are doing huge amounts of work recognising objects, performing hand occlusion etc. Again, something that would be very tricky and have a negative impact on a connected Mac
Finally, I can attest to how much it's essential for the Apple Vision Pro to be untethered. Even when I plug the battery into a charger, I find myself occasionally walking off and nearly pulling the thing off my head because the passthrough illusion is so good. I don't fancy pulling my Mac with me!