No idea. We need to know few things about pools, beyond doubt.
If mining pools collude, isn't that like stealing from their pool miners?
It depends on what they do with that collusion but generally no, it wouldn't be stealing. In fact, if by colluding they can get higher rewards that would be beneficial to their miners.
The point is that any criteria for the "bottom 50%" or "top 50%" has to be independently verifiable by any node and there doesn't seem to be any feasible way to do that.