I think Apple might be taking a slightly different and more affordable approach that is more compatible with existing technology.
Remember Unified Memory is about on SOC memory and sharing it between the CPU and GPU, not motherboard memory.
In this configuration you could regard the fast on-SOC memory as a type of cache, fronting (slightly) slower motherboard RAM. Introduce fast and relatively cheap PCI5e NVMe drives and a good virtual memory implementation and for a end-user device, you probably could remove the expensive motherboard RAM and just go straight to secondary storage - users for most of the time probably wouldn't notice a performance difference.
Obviously, you will need to make a judgement call on the life expectancy of the NVMe memory; suspect if it is greater than 4 years (ie. comfortably longer than the 2 year warranty) of typical usage then it is good enough.
Biting the hand that feeds IT
