Toothbrushes are not a 'solved problem' because a toothbrush is not a problem. Clean teeth is the problem (or cleaning teeth or not clean teeth or !clean_teeth

Toothbrushes are not a 'solved problem' because a toothbrush is not a problem. Clean teeth is the problem (or cleaning teeth or not clean teeth or !clean_teeth). Toothbrush is a solution to the problem, this is another solution. Just like you say 'solve flossing instead'. Flossing is not a problem, stuff stuck between your teeth causing decay is the problem.