Do musicians get carpal tunnel even after they quit music?

Yes: Carpal tunnel syndrome (cts) is a very very common disorder. It is more common in people who are active in certain occupations, etc. Sometimes the symptoms will persist, even after the offending activity is stopped. Some people have cts for no apparent reason. The treatment of cts depends on the severity of the compression, and on the severity of your symptoms.

The damage is done: Yes, if they develop it while they are doing the repetitive motion then it will stay with them.