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If the DC-DC fails, the voltage would take a finite time to fall. During the time it goes from 9v down to 0v, the mosfet driver would be off. So, the mosfets would be off.
Now, let's say we removed the undervoltage circuit. Let's imagine a situation where the dc-dc started to only put out maybe 4 or 5v for some period of time. During that time, the mosfets would quickly burn up, since it would take forever during the transition period. The PWM would still get through, but they would take forever to turn on, and a huge amount of heat would be burned up inside them.
If the DC-DC instantaneously fell to zero volts, then there's no cause of concern, since the mosfets would be off, because of the pull down resistor from gate to source.
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