when you see that picture you can start to understand why those extra seats weren't built, they will be a long way away and high up so, compared to the other seats, harder to sell. Hopefully we can squeeze 55k seats into the current footprint/roofline,It's the one we are building, the 60k went higher again. There is a cross section a few pages back (prob like 40 now how quickly this gets added to!) showing the overlap of the 60k version to the above drawing.
Edit: Here it isView attachment 175368