Still don't know why a smaller cable was needed. This card is the size of a game console, why does it need a slim cable? I would much rather buy some AIB partner 4090 that has four 8-pin connectors instead of one new connector.
And the new connector also adds 'sideband pins' so the power supply can tell the card how much power is available. That should reduce problems with random shutdowns due to drawing too much power.
With that said, a connector that melts clearly isn't an improvement.
No, most modern PSUs come with solid 8 pin connectors (as well as 6+2 pin connectors that you mention). If you watch the YouTube PC building channels, you'll see no one uses the latter for cards with 8 pin connector.
For reference, an Anderson PP45 connector pair (from 1966!) is smaller than the original 8-pin connector and would handle 660 W at 12V reliably at up to 10000 cycles (and can even handle hot-unplug).
Besides aesthetics and ease of integration in the chassis, air flow / cooling is a non-trivial issue, particularly with the wattages systems are running now. More or wider cables increase chassis air flow impedance which requires higher pressure fans to overcome which translates to a louder overall system.
In those kinds of cases, cable management and space/size tolerances can be very difficult - swapping four connectors for one makes a big difference