Enamel can erode for years and still protect your teeth until suddenly it's worn away and doesn't protect your tooth anymore. I got similar feedback from dentists while not having great hygiene and suddenly in one visit after years and years of "doing great" I had four cavities.

My girlfriend has been bragging in a similar way and I've been trying to warn her to be more careful, and she knows exactly what all I went though (fillings kept falling out, eventually lost the teeth and needed a bridge).