100% agree the problem is modular RAM can't handle the timings of the SoC.
There is a new modular RAM standard for precisely that but knowing Apple they will want to make their own.
SSD should be easy but since RAM does not last that much longer you still need to resolder that after 5-10 years!
you can have a mix- 4gb embedded ram + 1-2 slots of slower layer
Pretty difficult to code OS to take advantage of that. Basically need NUMA, which increases overall overhead.
Otherwise, you may end up filling up your fast memory with some cold data.
If apple cant support NUMA ill eat my hat