Erik:
The supervisor chip could be called the BEST way to drive the reset pin. It is however not the ONLY way to reliably drive the reset pin.

Goodness knows back in the days of 8048's and early 8051's there were no supervisor chips. It was also extremely easy to demonstrate back then that Intel's recommended reset R/C reset circuit was not reliable. As such there were quite a number of rather good reset driver circuits devised by clever designers. And it was these circuits that led to providing incentive to chip companies start making supervisor devices.

Michael Karas