I agree...sometimes you have to cut a prospect to keep vets. Cutting to 27 is hard enough.
Think about cutting to 15...forget about it.
You'd keep that 15 core vets, then redraft vets and prospects...only to cut most of them again the following year. The continuity of building a team is gone. It wouldn't work, and would change the character of the "perpetual teams" in a negative way.