As far as I understand about chip design (not much), the fact that the memory is inside the same package allows Apple to do stuff that would never fly with unknown external memory.
They know the exact latencies and can distribute the memory between CPU and GPU as they please.
A loss in upgradeability is a huge gain in speed and reliability.
My bet is that the next M processor will just have more of everything. More cores and more built-in memory. Maybe the one for the (i)Mac Pro will have upgradeable memory on top of the built-in ones. All of the laptops will only have the on-package memory.
They know the exact latencies and can distribute the memory between CPU and GPU as they please.
A loss in upgradeability is a huge gain in speed and reliability.
My bet is that the next M processor will just have more of everything. More cores and more built-in memory. Maybe the one for the (i)Mac Pro will have upgradeable memory on top of the built-in ones. All of the laptops will only have the on-package memory.