Most likely, there are many posts on (6v) starter problems because most old trucks have wrong gauge battery/ground cables. Add to that the higher probability that the 6v starter motor is older and more worn out than 12v starter motors.
Regarding a 6v starter having a shorter life when run with 12v, that is hard fact to argue against. Except that the amount of time needed to start a well-tuned truck with a 12v electrical system and a 6v starter should only be a few seconds of operation for each start.
