Did Boeing's engineers really think that carriers wouldn't eventually move to the maximum allowable density? I'm still bitter that my long flight in Y on ANA two years ago, booked 10 months out, was on their 788 fleet that was reconfigured hastily and suddenly from 2-4-2 to 3-3-3. Just awful, and I'm a fairly small guy. I would have gladly paid a little extra for the original config. Heck, I did try at the airport to pay to upgrade to their 2-3-2 PE cabin, but no dice.

Long story short, Airbus was smart to make the A350 just wide enough to fit 3-3-3 at a comfortable seat width, but too narrow to cram in another column. Boeing was naive at best.