Regarding UA E+ vs. AA MRTC, the crucial difference is that UA E+ seating is only in the first several rows of coach which AA MRTC was throughout the whole Y cabin. UA uses it to reward its full fare Y and elite pax; AA rewarded everybody, now they are punishing everybody. There are even reports that UA is offering some non-elite fliers on discount tickets "upgrades" to E+ at check-in for something like $25.
With AA moving to LRTC, UA's big hope is that the higher spending pax who can't get upgrades to F will move on over to UA, allowing them to obtain a higher revenue per seat mile. I am actually a little bit surprised that AA got rid of it entirely; AA does have more F seats available on their single aisle planes than UA, so maybe that's a factor.
Remains to be seen if it will work. If not, it will probably go away, in spite of it being the overwhelming #1 choice in that UA thread.