I definitely remember ordering a replacement headphone socket board for an old MacBook and just putting it in myself, and I've done similar with, eg, the connector port board on a PS4 controller.
But I guess this is one of the hazards of Apple having their AppleCare programme— they can do the math and calculate that saving a dollar on the cost of making and mounting and connecting that extra component still makes them money even if they have to eat the occasional full motherboard in the first three years of ownership.
https://www.ifixit.com/Guide/MacBook+Pro+15-Inch+Logic+Board...
You can see the ports still in place after the motherboard has been removed.
If you design to a socketed DIMM package, you actually have two trace domains, the CPU to the socket, and the socket to the RAM chips. For that to be reliable, you have to adhere to standards for DIMM (or SODIMM) so that multiple manufacturers can build compliant interchangeable parts.
If you solder the chips down to the main board directly, you only have once domain for the traces, CPU directly to RAM. At that point you can tweak the design for several goals, speed, power, etc.. And you can tune the drive strength on the signalling the same.
That is how you end up with more efficient soldered designs over socketed designs.
That said, I don't know that RAM chips are the hill I want to die on, anyway. Could we get to a better place if we dispensed with the whole, "just the one big motherboard," design, and looked to the old backplane-style physical layouts? Then you could have a module that contains the CPU and RAM and suchlike all in a single package, and move other things that may not want to be quite as tightly integrated off to another package.