The point is more that we can do experiments where we expect to observe interesting things or experiments to validate existing theories. If you have an experiment to validate an existing theory, it doesn't really make sense to repeat the experiment on a much large scale unless you anticipate actually being part of the first category (new observation, no theory).

The problem with colliders is that the next energy thresholds where we expect to observe unexplained phenomena are a couple orders of magnitude greater than the proposed collider. So we could really only confirm existing theories unless something major is hiding in the energy gap. This seems unlikely.

Critics want to put the money towards avenues where we're likely to observe new things instead of confirming validated theories.