UA/MP had to because CO's top tier was at 75K and not 100K. It would have pissed off CO elites who'd only earned slightly more than 75K but not 100K to suddenly go from top tier to mid-tier (50K Gold) with hundreds of thousands of others. AA will introduce a 75K tier when it merges the US FF program with its own because US has that tier and it's now the industry standard (UA, DL, AC).

But back on topic, if true PE was deemed viable and profitable for US airlines, DL or AA if not UA would have adopted it by now. None has and the other two have added their own versions of E+. AC has a very different market, and it has only adopted PE on two specially configured 777s because it adopted a uber-high density in the back. It's not converting the rest of the widebody fleet with this cabin. (AA took a slightly different approach on its new high density -- i.e. 10 across -- 777s and left a small E+ type 9 across cabin instead of a true PE cabin.)