I could be way off here, but I have been looking into this as well.

I dont think 6S is easier on an ESC. The only advantage of running 6S, over 4S, is you can go with a lower kv motor & still get the same performance, which, with the lower kv motor, would be easier on the esc.

the higher the voltage, the lower kv you can go. The lower the kv, the less amp draw you pull which = less heat for both motor & esc.

If I am wrong, I'm sure someone will chime in as I have been trying to understand this myself. I am considering going from my 4S / 2000kv setup in my truggy, to a 6S 1350kv setup.

EDIT: reracer beat me to the punch!