Its two issues with the 739ER, range (3235 nm = 3722 sm) which with prevailing winds is no where near enough for Europe to US, or many Asian secondary city routes, and marginal for US-Hawaii on many desirable routes, second even when fully loaded its not the same as an 753 which the airlines that have them have found is a handy plane size wise.

the "A321neo 97t" (97 Ton) is said to have a range of 4000 nm, which is slightly better than the current 752W (3900nm). My guess is that Airbus will find a way to push that a few 100 extra nm, which will solve the trans-alt West bound issues the 757 has had.

The 321 is a nice plane, with a 6"wider cabin it has 18" seats, the "issue" is that it seats 6 fewer in most configurations than does the 752, and my guess is that United wants an aircraft of the same capacity as the 753. That said, Airbus is not going to produce such an aircraft, and I don't think Boeing will (with the 321neo 97t already on the market) have anywhere near the market for a larger narrow body. United will have to buy these A/C at some point or use a smaller (788) narrowbody.