When a stylus is used beyond the point of its intended lifespan, it begins to develop flat areas, which can cause damage to the record grooves. This results in a significant and permanent loss of fidelity as well as an increase in surface noise. Contrary to belief, stylus wear is not possible to see under a microscope without laboratory grade equipment.
The image on the left shows two types of diamonds. On the left is a fresh stylus with no wear, versus a severely worn one on the right.