That old claim that sand wears out has been around for years, but I've never seen any evidence that it's true.

What does happen is that pools are build with undersized filters and oversized pumps, which (a) forces dirt THROUGH the filter and (b) causes sand loss on backwash. Check your filter's sand level. If it is low, (a) refill and (b) install a valve between the pump and filter and throttle the flow back, to maybe 1/2 of normal. Then, vacuum your pool.