You may have a valid point, but another failure point in starter motors is the bearings, and those are certainly not designed for sustained loads at all.
I'm trying to the mental math as I type, and this is not helping.
Watts=Volts X Amps if I am not having a blonde moment. (now that I am in my forties, these seem to be happening more often...)
So my brain is agreeing with your point in theory, but my gut is pretty sure that it wouldn't work for very long.
And now I need a nap...