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There are dozens of threads about this spread out all over, including people who have lost hundreds of thousands of miles. Check most any of the hotel or airline boards for discussion about that particular program.

Outstanding miles or points are a liability for the program, so they clear off some of that by making them expire.

Personally I fail to understand why it's so difficult to have some activity into a program once a year or once every eighteen months, I always feel that if I can't do that, if I can't have at least one mile post into an account, I probably don't need to keep that account.