Most switchers what to turn the POWER FET on hard (fast). You will notice the resistance on the input is high about 5K. This will soften charge / discharge of the MOSFET giving you a long time to get current flowing through the inductor and it will not work. On any open collector (drain) drives you pull current down hard and a resistor pulls it up. To Reduce power the resistor is a high value. You cannot turn off the power MOSFET with a high value resistor very fast. What the original design does is have a BJT 2N3904 turn OFF the power FET fast. The Drain on the 2N700 pulls the gate down (on) hard. This makes the FET have a fast OFF and ON drive.
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