If anything, from a mile-valuation in USD perspective, DL is the US airline that's gone the longest without a devaluation (and as long as they have the Pay With Miles escape hatch, arguably the one with the least ability to devalue going forward)... It's been a stable USD value now for 5+ years (and not impossible that a SkyMile is worth more USD now than it was then, given the ability to earn elite credit). UA has had a couple of brutal devaluations in that period, and AA some lighter-than-UA as well.