The GP comment can be misleading because it suggests Apple is tying storage to ram. That is not the case (at least not directly).
The RAM and system-on-chip are tied together as part of the system-on-package. The SoP is what enables M chips to hit their incredible memory bandwidth numbers.
This is not an easy thing to allow configuration. They can’t just plug a different memory chip as a final assembly step before shipping.
They only have two SoPs as part of this launch: 9-core CPU with 8gb, and 10-core CPU with 16gb. The RAM is unified for cpu/gpu (and I would assume neural engine too).
Each new SoP is going to reduce economies of scale and increase supply chain complexity. The 256/512gb models are tied to the first package, the 1/2tb models are tied to the second. Again, these are all part of the PCB, so production decisions have to be made way ahead of consumer orders.
Maybe it’s not perfect for each individual’s needs, but it seems reasonable to assume that those with greater storage needs also would benefit from more compute and RAM. That is, you need more storage to handle more video production so you are probably more likely to use more advanced features which make better use of increased compute and RAM.