technically we would still have mxm and socketed laptop cpu's if intel and amd didn't stop making them/ stop being supported. Mxm past 2000 series isnt really supported officially and the socket g3 was the last socketed cpu that was mobile. The desktop Cpu's have different p states than the mobile chips do. So those would have to be made again. The only thing which i dont like about past laptops was opening them up was a pain, since it wasnt as simple as opening the back.
Soldored ram was done as when you get laptops where size is really constrained. it gets tough to do so. especially with bezels getting thinner, allowing for bigger screens at a lesser size. Wifi, i do not get at all.
Problem is that even if it does get revived again, it will die quickly unless some big oem or intel/amd set some mobo standard like we have itx/eatx/atx. Forgot to mention another thing, older laptops with upgradable comopnents could still effectively lock you out with bios updates. cuz compared to desktop mobo's not every mxm gpu is compatible from the getgo, you need a bios upgrade that supports the new gpu generation.
Look at the 51m for example. Newer 51m wasn't necessarily upgradable due to the older ones not supporting the newer cpus or gpus.