How is that supposed to work in practical terms? It seems that you are suggesting one of two things: a) that all Macs will ship with max RAM configs and you can pay to unlock then down the line or b) that RAM itself is modular.
The option a) is not feasible because of overall scarcity of high-end RAM Apple uses. It would be a bad business move for Apple, as it will significantly impact their ability to produce these machines and cut into their revenues. Not to mention that it will likely cause the reverse effect — people will be less likely to spend money on RAM upgrades since they can do it down the road anyway — and if they don't do it at purchase, they are probably never going to do it at all.
The option b) is not feasible because there are no efficient mounting options for the types of RAM apple uses. I mean, they could potentially use an LGA-type mount with hundreds or thousands of pins for their custom RAM, but that will be expensive, take up a lot of space, fragile, and likely kill the power-efficiency of their mobile solutions.