LPCAMM2/SOCAMM2 exist, heck I think Framework is using LPCAMM2 in one of their new laptops.
Heck, I'm willing to bet that a lot of manufacturers would rather go that route than soldered in, if for no other reason than the relative cost of warranty work between the two.
However, people probably need to stop being obsessed with ultrathin laptops for that to happen.
I've never been able to understand this. Once we made it down to ~20 mm (which for the record still accommodates dual-stacked SO-DIMMs, a 2.5 inch bay, and a user replaceable battery but not an RJ45 jack) I don't understand what the practical impact of any further reduction is supposed to be. Regardless of how thin you make it the thing will still be a massive rectangle that you can't flex or press on.
There's very wide variation between laptops in how noticeably they'll flex or yield or creak when pressed. Laptops with a build quality that actually feels solid are far from being ubiquitous or even a majority.
Doubling the thickness of my MacBook Air would probably make it regress on that solid feeling, unless the weight was also significantly increased.
And regardless of whether current laptop form factors could accommodate a 2.5" drive, there's no use in doing so. That drive form factor is entirely obsolete for laptops and is just a waste of space and materials, and has been for about a decade.
I'm not sure why you seem to think that making something thicker would reduce the stiffness or strength. It's generally the opposite - see the concept of a torsion box. Anyway that wasn't the point. The point was that regardless of how thin you make the thing it will forever remain a cumbersome and delicate item that you have to treat with care when packing so what meaningful positive impact does shaving off those last few mm have? It's never made any sense to me.
I would much prefer two SODIMM sockets with the option to go to 32MB shared video memory, or DDR4/DDR5. Give me OPTIONS!
Unified memory doesn't have to be soldered on or serviceable. That's a choice Apple made because it fit their product vision, but it's not mandatory in the slightest.
CPUs don't slot in for a reason