Sorry, but everyone is wrong.

The #1 reason why UA cannot take the extra space of E+ and spread it across the entire cabin is because of the fixed location of the overwing exits.

AA's now defunct MRTC was essentially an E+ in front of the exit row and another E+ behind the exit row. UA is E+ only in front of the exit row and not behind the exit row.

You can't remove half a row in front of the exit row and another half a row behind it. Rows are integral.

I don't remember if the large planes such as a 747 had more than one row removed for E+, but this is certainly the case with the narrowbody planes.