I don't think the conclusion follows.

It's entirely consistent to believe that the level of something is too high and has been too high for a long time, yet to not oppose it in principle.

The correct question to detect if that's really their objection is not "have they ever thought that the level is too low"--the correct question is "would they ever under any circumstances think that the level is too low". Of course, you're not going to get as many "no" answers with that as with your original formulation.