I have a Polaris 360 and worn plaster but I'm sure it didn't cause the worn plaster. You're in the same boat I am--you're not sure how old the plaster is. I know my pool was built 23 years ago and I have some idea about how diligent the other owners were about service (or weren't). The P360 is a very good cleaner. A new plaster job seems to run $3k-4k. Not keeping the water in balance can contribute to plaster deterioration. Sounds like that pool just needs the loving attention to detail that it will start getting!

Good luck!

Hal