Dear Dr. Ellie,
Do you think that natural bristle toothbrushes are less abrasive than nylon?
I think it is more important that your teeth are not acid-softened when you brush them, and that your toothpaste is non-abrasive.
I do not think you could abrade tooth enamel with either kind of brush providing it is well designed. Rounded nylon tufts are important as is their hardness/stiffness.
Both kinds of brush get infected easily I wonder if natural bristle is worse for this than nylon (no studies that I am aware of).