I think it's safe to assume that all carriers will devalue miles over time - increased mileage requirements for award tickets and so on. Frankly, unless someone flies so much that they just can't use the miles as fast as they earn them I don't understand why anyone would want to amass a million or more miles in a single carrier's program unless they had a specific use for that many in mind, like around the world tickets for a family of 4 or more.

Jim