all self commutated brush type DC motors have a maximum no load speed regardless of winding type
the back EMF is directly proportional to RPM, so eventually the two will reach a balance after losses and the motor will settle to a steady state RPM.

i will try to explain,
when the brushes swipe over to the next bar on the commutator a small portion of the rotor winding must change current flow direction
this change in current flow must increase in frequency directly with RPM
the impedance (resistance) of an inductor increases linearly with frequency Z=jwL, or Z=2*Pi*F*L
so as RPM goes up, the inductance of the rotor looks like an increasing impedance
this is why the starter current drops with increasing RPM, which means the total power drops with increasing RPM

at steady state your starter spinning in the vise will only be consuming on the order of 400-500 watts, that's 4-5 100W light bulbs worth of heat
so how long can a big block of iron like a starter absorb about 400-500 watts before getting too hot?
that's how long you can run that starter non-stop without appreciable damage
but it will be wearing out the brushes like mad
but wearing them out at a constant RPM, not an increasing RPM


there is one type of motor than can increase RPM unbound under no load, but it is never operated in that way
an induction motor with a separately excited rotor capable of synchronous and non-synchronous operation will present the line with a negative impedance if the rotor is left OPEN,
in this situation the induced voltage on the rotor winding will increase in voltage unbound until the line load presented causes circuit breakers to open up.
if the circuit breakers do not open up in time the motor will experience arcing across the slip rings or through the winding insulation in the rotor because that rotor voltage will be going up up up!

usually the rotors do not throw their windings in these cases, the machines have only a few 10 thousands of an inch clearance between the rotors and stators, this gap is made as small as possible to increase efficiency, due to vibrations and machining tolerance limits the rotor will begin to crash into the stator and become quite "turbulent" this crashing and friction will produce an amazing display of sound, molten bits of metal and sparks until the whole assembly finds a way to come to a flaming stop
-s