The usuall problem is not that the syncro is actually worn out. If you look at the pic you will see a metal piece sticking out of the gear about 1" long. That is the "insert spring" That is what puts pressure against the syncro from the sleeve moving while it is engaging in gear. If the springs are broken, worn out ect you have no pressure on the syncro when you shift and the syncro will not speed up the gear so it does not grind. If it does not speed up the gear to get to the same speed as the last gear/shaft it will grind.
I bet that if you were to measure a grinding trans gear/syncro gap you will come up with it being in spec. Usually the hub/gear is worn and the insert springs are trashed. When you get to much gear end play it breaks the springs.
