I think AA proved a couple years ago that taking seats out for more legroom does not work. They only let that go for a year or 2 before they realized they were throwing money out the window.

In a perfect world I'd like to think that people will pay for E+ but I doubt they really do. Even if they do, it's not that much of a fare premium. Just randomly looking at SEA-IAD in 2 weeks on UA puts you at $899 round trip. For E+ they only want $49 more each way. There is no way they are making more money by having E+ instead of putting in an extra 2-3 rows.

Air New Zealand has been quite successful with their premium economy but that is purely on their longhaul flights. I think people are willing to pay more on a 12-14 hour flight than they are on a 5-6 hr one.