Two points in response:

1) JonNYC has speculated/revealed that UA is considering retrofitting widebody Polaris seats with privacy doors and/or other enhancements. You also have the new Polaris seat under development for the 737 MAX 10s and A321XLRs, which is expected to be competitive with AA's (and B6's) new narrowbody seat. I can't find the tweet where he mentions this, but trust me, the rumor is out there, almost certainly in response to AA/DL adding doors to their seats.

2) It may have taken a long time to get there, but UA is working towards product consistency--all widebodies should feature the same Polaris seat within the next year (depending on the pace of 764 mods). In contrast, AA has an inconsistent product now--you have three widebody seat types, with one of them decidedly inferior to the other two and Polaris IMO--and they plan to maintain that inconsistency moving forward. The possibility of ending up in an inferior seat when booking on a 772 or 788 is a huge deterrent for me. DL also has a product inconsistency issue.

JonNYC has tweeted that he doesn't expect the 772s, which are split roughly 40/60 between the inferior and superior seats, to be retrofitted at all: