They could have derated the 757 as per the 777X, and made up the lost thrust with aero tweaks; there have been a lot of developments since 1980. Also the blueprints for the shorter 757-100 should still be in a locker in Renton.
But that wouldn't have addressed the KPI of the 737 Max: commonality and type rating with the 737NG. That's what drove the whole program and which led to hack upon hack. Southwest would not have bought a 757 Max unless Boeing could price it well under the 737NG, to compensate for the additional operational costs. Southwest wouldn't even consider a clean-sheet design!
Back in the day we used to shake our heads at British manufacturers tying their specs to the whims of BEA and BOAC, but Boeing seem to be falling into that trap with the Max and the 777X ( Emirates-pleaser ).
Anyone writing a history of Boeing in the future will be able to conclude the first section with the 777 and 737NG.