Delta has their version of United's E+ but it is fewer rows that UAL. American does, too.
The airline market is very efficient. The reality is that most consumers will not go past the fist page of an internet search for an airline ticket. So when your fare pushes you past the first page, you've lost customers. The industry has tried several things to improve passenger comfort, all of which have been busts. Time will tell if economy plus will stay around.