I'm not sure they can make RAM upgradeable at this point and still keep their unified memory. If you look[0], they have the memory under the heat spreader, butted right up next to the SOC, and my impression is that this is what allows them to get that nice memory bandwidth.
[0]: https://www.ifixit.com/News/46884/m1-macbook-teardowns-somet...
Apple even has a patent (garbage one, describes something obvious) for second non POP slightly slower ram channel.
https://www.techpowerup.com/277760/apple-patents-multi-level...
"With two types of DRAM forming the memory system, one of which may be optimized for bandwidth and the other of which may be optimized for capacity, the goals of bandwidth increase and capacity increase may both be realized, in some embodiments,"
They are sitting on a solution while extracting value while market bears it.
And they could combine onboard unified RAM with a slightly slower non-unified expandable RAM slot. There is some precedent for such configurations in computer history.