They would not have a leg to stand on if they put up the soft drink tax and then not implement better dental care as the medical/dental profession and the rest of us are seeking, but I understand where you're coming from.
Worse case scenario: the govt doesn't do anything to improve access to dental care, there would still be positive health outcomes. Overseas they've found that putting up the price has reduced consumption of these drinks which in turn has reduced some obesity and dental caries, particualarly in the young.