It would kill the common cockpits with the rest of the family. And starting now would take years for the software development and more years for certification. Still it is Boeing's job to stick to the rules and to not base everything on grandfathering extensions. How could they end up in a corner like this?
Maybe they just should forget the -10, double the -8-rate, halve the price and flood the market with it while Airbus is sold out on the A321neo? And start to design the next family from scratch.