Odds of Hanging On
Tank Commanders will eventually move on from the position. If a Tank Commander performs well, they will be promoted, possibly to Platoon Leader, and leave the inside of their tank.
If they don't perform well, they may be discharged. There's also the very real chance that a Tank Commander may suffer some type of injury that requires them to leave the military. Combat soldiers also have to face the possibility of developing Post Traumatic Stress Disorder, which could boot them from active duty as well.
Most tank commanders, even those who see heavy combat, don't have to worry about these issues, however. They will either move on from the position into another military job or they'll leave the military when their tour of duty is up.