What are the impacts of a worn pizza stone?
Pizza stones are typically either:


Refractory ceramic with a high thermal conductivity, moulded into a useful shapes. Fired, but unglazed
Diamond saw cut slabs of suitable natural stone


Either type will respond well to heating and cooling in a normal oven. And take reasonable amounts of general wear and tear

Both types will chip on the edges if whacked, and will develop cracks over time that will cause it to split, normally only into two pieces

If they crack during normal use, and are fully supported by a metal oven rack, they will still perform fine, and may even continue to be used in the future

Warning: Natural stone that has been submerged in water for a long time may explode when next heated in the oven. But normal washing and care should not cause this situation

Note: According to many recent studies and famous books like "Modernist Cuisine", a pizza stone is not enough, you really need a large slab. e.g. a 10 kg steel sheet