Mini-Fit does not melt at 100W unless significantly damaged. Molex's own spec for eight circuits onto a PCB is 10A per circuit at 16AWG (120W) for 30C temperature rise over ambient. You could run 360W on 3+3 pins before you even brush up against the safety margins which, again, are given with a third more heat since they assume all eight circuits are carrying load. Conversely, I've seen cards with two 8p connections burn one up at under 200W loading, because the receptacle was bent and didn't make proper contact.

Mini-Fit is so insanely overspecified for the requirements of a PCIe unit load (which is why PSU manufacturers can offer Y-cables that use the same physical connector for two unit loads without issue) that it could have worked. The problem is they went entirely the other way and ran the Micro-Fit connector as close to the edge as they possibly could, then sold people adapters that introduce design elements not considered when Molex designed the thing. What they should have done is just shrink the PCIe 8p connector to Micro-Fit and take the opportunity to repurpose the wasted main pins from compatibility with the 6p, so that it would have an actual 4+4 power pins. You'd get a 30% reduction in size from the switch from 4.2mm to 3.0mm pitch and a 30% increase in power from using the fourth pin, for a total of 80% increase in power density and still tons of headroom for unknowns. At that point, the area occupied by the connectors would be no issue at all. But they decided to go for that 270% instead, and here we are.

Toyota did it first.

 

Over a decade ago, too.