Don't criticize airlines for segmenting their product. This is a basic business principle: Airlines want to sell customers the most expensive products they're willing to buy.
Upgrades to American's domestic premium economy (Main Cabin Extra) foreshadow a better experience for elite travelers — but also the potential beginnings of an entirely new class of service.
United is lagging behind its competitors in launching international premium economy. But this is something it needs to do, if only because of competitive dynamics.
A surprising number of airlines held out against adding premium economy as long as they could. But now most big airlines have added it or plan to. That's good news for passengers who want a little bit more space at a reasonable price.
Delta just went public with the design of its premium economy product, which isn't too dissimilar from competitors' features. Expect to see Delta's in service in late 2017 with the A350.