There are many things that cause cavitation, debris, snails, broken snail shells, 1 grain of sand in the perfect position to cause the impeller to wobble and wear.
I had this same kind of wear on a Mag Drive 24 after about 5 years, you have to make sure the wear has not also worn a groove in the casing or the volute, if not, a new impeller is worth getting.
Note the almost figure 8 wear inside the casing above , where the impeller wobbled in this mag 5 and eroded the casing away .
above the skin on the impeller was peeled away, probably from sand.
I tried play sand years ago, and I believe it got into both of the damaged parts above.