Honestly I just don't want them phased out. Give high turn people (4 or greater) ambush, unless like with Sand Anbu, some effect gives it that based on some cir****tance or condition.
Only T6 super high ranked masters should have Mastery. The only exception is a ninja with some sub-par lame effect or no effect at all (IE there was a jiraiya that literally only had Mastery and Senjutsu. Using any chakra for jutsus + being able to mastery them back is pretty beast on it's own for a T5 common)
Lastly, Growth should ONLY be given to ninjas that have o PiP effects (except maybe getting some effect with "if this ninja is PiP by growth..." effect)
Follow these rules and maybe 10 ninjas max with Growth (thats at least 2 per element) and the keywords would be fine.