Yes, of course. It would be eminently impractical to try and make a vehicle around supercaps at the moment. That's not the point I was trying to make, though. I was trying to point out the systematic error in thinking.

You've correctly pointed out that the supercaps are too big and heavy and such. But let's just imagine for a moment that they're made of magic and are 5% of the volume and 5% of the weight of batteries. It's still not a good idea to charge them up to then charge the batteries (from fixed charging stations) because then they're better than the batteries full stop. Throw away the batteries and use them the end.

The only way that they make sense is for what you've described, they're electrical buffers to help impedance match "slow" batteries with "fast" motors. But since that wasn't what the parent was asking, I didn't answer that question.

There will be a physical limit when you attach the cables to the capacitor. At some point, the cable will be larger than the capacitor itself. Also, there are internal components (i.e. the leads attaching to the 'plates') that will become limiting factors. All that to say, there's a physical limit to how much bigger you can make the cables and actually still attach them to the capacitor.

Not for supercaps, most have a breakdown voltage below 5 V.