You open your frequent flyer account and see 150,000 airline miles waiting to be used. It feels like a valuable asset. After all, you've spent years earning those miles through flights, credit card spending, promotions, and loyalty programs. Then you search for a flight you've always wanted to book. Instead of the 60,000 miles you expected, the airline now wants 95,000. Another route requires nearly double the miles you remember seeing just a year ago. Even economy tickets seem to cost more miles than before. Your mileage balance hasn't changed. But its purchasing power has. This is the hidden reality many travelers don't realize until they start planning a trip. Airline miles rarely lose value because miles disappear from your account. Instead, they lose value because airlines gradually change what those miles can buy. Understanding why this happens can help you make smarter decisions about earning, redeeming, and even deciding whether to continue holding large mileage...