most of the time things are soldered into place. this is deliberate to stop laptops being upgradeable.
I think you are too paranoid here. I would say to make it modular, you need connectors, which increase the size and weight.
For GPU, the laptop is designed from the start with a certain cooling power, so making it replaceale would mean compromising too much the cooling (as in making the cooling solution more adaptable). Also how do you prevent the user from keeping a mid-range cooling solution and putting a high-end GPU?