Wonder when we will get GPUs that treat the PCB as a motherboard.
I'd speculate; never. Different GPUs have different power requirements, substrate sizes (IE, number of connections) and so on. The one-size-fits-all would be the biggest size for everyone, and mid/low-end GPUs don't need 300+ watt voltage regulators. It's just increased expense for no benefit.
If GPUs are to be replaceable, you'd reasonably need to cap them, for the sake of durability. This adds thermal resistance, vertical height, and cost. Sockets also add vertical height to the board, making it more difficult to fit a decent cooler.
You'd also get locked into a socket layout for an extended period, dampening innovation, or else facing rapid obsolescence. Like with complications like NVLink for example, you can't really foresee something like that and pre-allocate connections for it, in case you need it. When you do have a need, you have to redesign the socket anyhow, so what's the point of even making replaceable GPU boards when they have no chance of lasting any significant time on the market?
Also, socket connections tend to be detrimental for high-speed electrical signals, it might not be feasible.
