Why wont my starter disengage after my bike starts

safety switches that require for you be astride the bike and the clutch disengaged
stops anyone from hitting the starter switch when on a stand and in gear and the bike taking off and doing damage with no rider

Starters have a "bendix", a system that moves the gear on the starter onto a gear on the engine, then disengages it after the engine starts. This is probably bad. Be sure you have a charged battery and good connections, low voltage sometimes causes this.

sounds like a stuck starter solinoid ! replace it !

Replace the ignition switch

The teeth on the flywheel or starter drive could be damaged enough to cause sticking and it does not release. Another reason could be the clearance between teeth on the flywheel and starter drive are too tight. The starter would need more metal tabs or spacers to increase the clearance distance between the teeth on each one.

You'll need to figure out if it is an electrical problem or something hanging up in the bell housing.
You can leave off the wire from the key and try it to see if something is powering the solenoid after you release the key switch. If your model has the solenoid on the fender it could be sticking.

The Clutch on some bikes will disengage the starter motor. Yamaha will not allow you to engage gear when the side stand is down. Hope this gives you some idea

Without power to the clutch and trans alot of bikes will not disengage. Only when running

you are compleatly right your starter has probaly started this bike a million times and its just getting tired now. replace the starter and enjoy