Mergers create a brief window where customers are largely protected from devaluations. Airlines are distracted by merger integration, and don’t want to make customers feel like the merger will be bad for them right away. Once American Airlines moved to a single reservation system in October 2015 we learned about a gutting of AAdvantage the next month both for elite status qualification and benefits and for their redemption chart, and then the rollout of basic economy the next year.
Alaska Airlines right now has by far the most lucrative US airline frequent flyer program. Their award chart is far more attractive than that of their competitors. And they’re a better airline to fly. That may change.