As far as I understand, fluoride helps in hardening the enamel of our teeth by replacing the calcium element in hydroxyapatite to convert it in the stronger fluorapatite, like explained here. Wether that's worth other potential risks of using fluor is debatable, but let's keep that aside. I'm curious if it is still beneficial to keep using fluorised toothpaste if you already did so for like 10 years. I suppose all hydroxyapatite is long converted into fluorapatite already after so much brushing. Why would it still be recommendable to keep using it?