Dissolution is based on concentration of existing solute. This isn't a binary effect of complete saturation like you suggest. Pure water has enough diffusion pressure to cause significantly more dissolution of a tooth's mineral lining than hard water. Diffusion pressure of pure distilled water is 1236 atm.

See DPD: https://en.m.wikipedia.org/wiki/Suction_pressure

Quote on quote: "If some solute is dissolved in water its diffusion pressure decreases. The difference between diffusion pressure of pure water and solution is called diffusion pressure deficit (DPD)."