Whatever Boeing does, they are going to have to absolutely knock it out of the park efficiency wise. Airbus is going to be able to sell 330NEO's and 321NEO LR's for a song as the development costs were paid for a very long time ago.

I think Boeing is going to lose this one They have dug themselves into a 737 hole, and they can't figure out how to get out.

As far as single or double isle, double isle isn't any good until you get to 9 across. The 767 was 7 across. For 1 extra seat per row, you also got an isle. The 767 was 50% heavier than a 757, for 16% more seats. That was not a good tradeoff. There will never be another double isle aircraft with 7 across seating in coach.