AC is going this route on an interim basis to test the waters on two 777s, not fleet wide. It is reducing pitch and going 10 across in economy, so to appease critics it's providing this new in between Y and J cabin. AC's also made elite upgrading very restrictive.

E+ is too valuable for UA to change it. And now that DL and AA have adopted similar seating there's no chance of change. It, and other US carriers, substitute generous upgrading to C/J for elites over the true Economy Plus concept.