A pool can become "invalid" if the ratio of points to balance becomes too high. (This is because it increases the risk of saturating arithmetic - more details in the pallet docs.)

There should be a permissionless extrinsic that allows the destroy process to start (stop nominating and block any new joiners, etc) if the points to balance ratio becomes too high. In the future there may be other things we need to check as well.

Should wait until #10694 is merged to start working on this.