Is there enough life left in these 752s for United to do an upgrade to direct aisle J, matching the Max 10s and XLRs?
Might be a little late for that seeing as UA is instead just refreshing the existing Diamond J seats with new fabric covers and new Y seats.
I think the jury is also still out on how well the Polaris seat might fit into a narrowbody, if at all. It could end up being a tight fit or not be space-efficient. Wasn't there also a leak on here (maybe more than a year ago) that showed the 737 MAX 10 getting J seats similar to the new B6 A321 LR seats? Those seats look nice, though they also look pretty tight, and the angle away from the window is not ideal.
The issue with Polaris is width of the 757 fuselage and fixed locations of Door 1 and Door 2. There is just not enough length to put in 16 Polaris seats. Maybe 12 would fit to replace 16 Diamond.
The only way to pull it off would be to clear the entire area between Door 1 and 2, but that would take a least 2 to 3 rows of Y to move galleys and a lav and still might not fit 16 Polaris seats. Maybe #2 below could work with removing closet, lav and galleys. However, this is probably too expensive to do on an older aircraft that is being replaced internationally with a 321XLR and could serve the Transcon/HI market for several more years as is. There would have to be a detailed redesign of the Polaris seat to fit into the space on the 752, even more expense.
Let's look at the 777 vs. 752:
If you take half the width of the 777 you would get 2 sets of Polaris seats and an aisle in a total of 115.5 inches (half of the 777's 231" width).
The 757 has a width of 139.3 inches, which means there would be 23.8 extra inches of width left over. Seems no way to utilize this extra width IMHO.
Let's look at maybe comparing to the narrower 787 for a couple options:
If you take half the width of the 787 you would get 2 sets of Polaris seats and an aisle in a total of 108 inches (half of the 787 216" width).
The 757 has a width of 139.3 inches, which means there would be 31.3 extra inches of width left over.
1.) To add a single middle seat (straight forward) it would take at least 43 inches for the seat and a second aisle, so that doesn't work.
2.) It might be possible to use the extra 31.8 inches by rotating all seats a bit so that the feet from the current straight seats are next to the passenger's head in front of them. However to use the same space as the Diamond seats, it would require a reduction of about 20 inches of pitch per seat set and that would undoubtedly make the seating too tight. In fact, at that pitch the seats would look Diamond seats with walls and no all aisle access.
Maybe a slight rotation could work with removing lav/galley/closets to utilize the space to get in 16 Polaris seats.