2, 2, 2 seating is much better than 3, 3 for longhaul. You get two isles so get a feeling of space and you only ever have to climb over 1 person to get out of your seat.
Yes, but the space costs. Boeing 767 has only 1 extra seat compared to 757 in normal economy, but the fuselage width is 127 cm wider (376 and 503 cm) and the frontal drag, skin friction drag, weight and induced drag are accordingly bigger.
30% of the middle seat can fold up, as indicated at the picture. That creates about 10 additional cross-over's. when two stairs / gates are used this should really speed up the process and short haul / leisure operations.
You could do the same on short-hop 767s. Is it done?
The single aisle variant offers superior seat comfort. The middle seat offers an unsurpassed 23 inch (including armrests) and the aisle window seats a still very good 21 (incl armrests) compensating for long flights in a relatively narrow cabin
No, it is not unsurpassed. Greenliner is plainly narrower than Sud-Est Armagnac, which has 470 cm fuselage width (compare A320 at 395 cm and 767 at 503 cm), and is single aisle with 3-3 seating.