All the 10K pullup does is hold the MOSFET "off" when the NPN is off, and bias the MOSFET
gate to approx -12V when the NPN is on.
You missed the most crucial role of the pull-up: it switches the MOSFET off when the grounding transistor closes!
With a 10k pull-up, you're going to be running a little over a milliamp into the gate of the MOSFET.
That means it will take a while to switch off.
That means it will spend more time in the dangerous "intermediate" region when it will have higher resistance, and thus generate lots of heat.
As I said above: For a single switch (duty cycle of minutes, say,) this is probably fine, unless the load is very high-current. For faster duty cycles, or heavy loads, you may find yourself blowing the MOSFET on a single on-to-off transition. I know from experience, where I had to go to a 100 Ohm pull-up before my P-channels would stop failing 
(That was with PWM, though)
The gate of the MOSFET is a capacitor. The pull-up is a resistor. Thus, you have an RC filter, and can calculate the rise times!
For a 10 kOhm pull-up with a 1000 pF gate charge, you have a 99% rise time of about 45 microseconds, which is actually a lot! With a 1 kOhm pull-up, you're down under 5 microseconds, which is better.