They could have easily made the RAM and Storage user serviceable/upgradeable

Nope, they couldn’t have, since the A18 pro chipset doesn’t support modular memory at all, of expanding it past 8gb. It’s a phone chip after all. There’s also the fact that Apple has equipped all their devices with unified memory, which, if they even managed to make it upgradable, all chips would need to support massive memory bus widths to have the same or similar bandwidth (requiring more modules), would need proprietary modules or at least rare modules like SOCAMM, and would reduce the space inside the chassis for anything else, like battery, modular ports, etc.

Sure, I hate Apple’s antics in terms of lack of right to repair, but frankly they produce arm based computers, where have you ever seen an arm based laptop or mini pc with modular RAM? I’m sure some exist but they’re likely too obscure for me to have heard of (although I have heard of System76’s Thelio Astra, although again they are a bit obscure outside Linux circles.)