I don't think E+ is a significant moneymaker or other airlines would have introduced something to compete. AA tried for awhile with their "More Room Throughout Coach" product awhile back where every row had more pitch but after some financial analysis they put the seats back in because they weren't getting a revenue premium for it. And UA just borrowed money at 17 percent interest, which tells me that they're not getting much if any extra revenue from E+.
http://www.reuters.com/article/marke...0090629?rpc=44
I don't see how MRTC is a valid example. What that experiment proved was that AA wasn't able to gain additional business accounting from MRTC to offset the losses in revenue. That's a very different thing from E+.
As for other airlines introducing something similar, what about B6?