It shouldn't carry over each season, it should reset every year, just say Cayless picks 10, it doesn't stop a younger player picking 10 when he is the starting prop.
Also, in the case of Lockyer, he would have been number 1 for the start of his career and will get number 6 now. This would keep most people happy but if Lockyer got injured and didn't play the rest of the year you would have a player with a different number in there.
Imagine the young number 23 on debut then next year getting the number 6. It would be like a reward for achievement. Tim Mannah would be aiming for the number 10 and working hard to wear it.