Almost all airline's miles expire EVENTUALLY if you have no activity (say, for 3 years). For a true frequent flier that sticks to one or two airlines (and their airline partners), that's typcally a non-issue. But I think the point of the poster you were replying to was that a vast amount of miles are awarded to INfrequent flyers who sign up for FF programs "just in case" but then forget about them and/or never have "chance" to fly that airline again before the 3-year (or whatever) expiration period is up. (These are people who don't choose the airline based on the FF program they belong to, they choose solely on price and/or convenience, and only earn miles if they "happen" to fly again on the same airline. Thus "by chance".)