you will be stuck forever if you insist on backwards compatibility ... keep in mind that we are still using ~30+ years old design for power delivery ...
and you dont need 40 pins in new designs ... even for 1300 watts ... the 12V per cable should be history !
This is a layman view of how electical engineering and power supplies work. Motherboards already pass insane electrical current for CPUs (albeit at low voltages and spread across hundreds of Vcc and GND pins).
You are saying that all current for every device inside the PC should be routed through the motherboard and you are totally ignoring:
1. Mechanical stress of adding new connectors to the board (not to mention space constraints)
2. Increased board thickness, trace sparseness, and better heat dissipation required to run all power through it safely
3. Effect of more current running through traces (EM, heat, crosstalk, material migration, to name just a few)
4. That board is akin to a bunch of exposed wiring compared to the insulated cables you run from PSU to devices
5. That board is now another unnecessary point of failure
Laptop is different from a desktop PC in so many ways -- for starters it wasn't meant to be opened and tinkered with so the integration is tight. In a desktop PC that's totally impractical idea.