Keep in mind that at most airlines, a lot of miles expire without being redeemed. That's especially the case with many overseas airlines which have a "hard" expiration (ie, you can't take any action to extend it). I don't know what fraction of miles are thus never used, but it could be a big fraction. Most people who earn miles are not on FT, so you can't assume that they all pay attention to FF programs to anywhere near the same degree that the typical FTer might. I have lots of friends and family who let airline miles and/or hotel points expire. Any one such person may not have a lot of miles, but it adds up if there are zillions of such people.