Within a club all players on the senior list undertake the same workload.
Therefore differences in payments are based on ability, the competition they play in and you value to the club. If you play league you are considered of more value to your club on that afternoon in comparison to a reserves player, so you get paid more.
Some players will leave for more cash/opportunity while some won't.
Increasing the pay of reserves players won't change that - the character of the individual will determine how long they will stick it out to get a shot at league footy.