[mpreda]

Now I must use lots of rather thick cables in my desktop (because I run GPUs).

Imagine that the GPU would instead suck up all the power it needs through the PCIe connector, without all those pesky cables. (right now PCIe can provied 75W at 12V, i.e. 6.25A; that same current would provide 300W at 48V).

[masklinn]

The PCIe slot would not be sufficient even if the power architecture moved to 48V: the 12VHPWR are getting 600W pushed through them.

[anticensor]

Card-edge connectors are wattage-bound, they would still be limited at somewhere about 100W if upvolted to 48V.