They wear at different rates depending on a variety of factors, but no doubt about it, they wear.
You ask what a worn diamond tip looks like. When examining for wear you do not look at the tip as such. You look at the light reflected off the sides. This requires a special set-up with high intensity lamps. The difference between a new and worn tip is obvious. When new, the reflection is a dot of light. As the tip wears the dot gets larger and defines an area worn flat.
Examining a tip at low magnification is for revealing general condition. You can see if the diamond is chipped or cracked. It happens.
When most tip shapes wear, the edges of the worn flat contact area become sharp and can tear up a soft vinyl groove just like a chipped diamond.
The synthetic rubber used in the suspension does not normally wear out. Suspensions usually collapse due to mechanical failure.