@ Victorian...I think you are mistaken!
A normal P/E starter- pinion has straght splines along which it slides to engage/disengage. The pinion has a one-way clutch (see my contributions to the "engine-starting" thread, for a full explanation.
the pinion stays in engagement whilstthe solenoid is energised. As the starter is geared at around 30:1 (pinion is about 4cm diam. witharound 8 teeth ring-gear around 50 cm dia . Typically a starter will crank an engine at about 80rpm and when the engine starts, it will tick-over at around 600-850 rpm. If the starter is continually engaged, the overrun-clutch will still transmit power...from the engine to the starter.....starter is not built to do10,000 rpm plus.....they literally throw their windings from the armature by a combination of centrifuging and the com-solder melting...welded coms have disintegrated!the segments flying off ,still attached to the winding-tailsand destroying the brush-gear and anytinig else in range.
A stuck solenoid is not that common, a more likely suspect being the key-switch or a relay with the contacts "arc-welded" together....also can happen with a low/duff battery (volts low, current high.)
the reason old cars wear the ring-gear is due tio the engine always stopping with one cylinder coming on compression....on a four-cylinder car, you'll see a worn ring-gear has four distinct wear-slopes around it's periphery the most heavily worn is where the engine always stops, so every start , the pinion smacks into those same teethand grinds away to pull that cylinder over compression....then the load eases until the next one ....that's why the wear is progressive....split a ring and straighten it out and it 's wear-pattern is four ramps...eventually, the teeth become that worn that the pinion doesn't travel far enough to engage...a new pinion can be a stopgap fix!
P/E engagement forks are invariably connected to the solenoid-plunger via spring-loading, so even if the pinion doesn't mesh, but hits a tooth on the ring, the plunger will still move against the springs and close the solenoid main contacts.....as soon as the starter motor turns, the pinion will go into mesh...again, it's very rare to get a jam.