Some of Boeing's large customers would only commit to buying 737s if the update did not require training or updating the type rating. The 737 was originally so low to the ground to enable it to use air stairs and operate at airports without jet bridges (most airports at the time of its introduction). Not sure anyone could predict engine efficiency would require engines so much larger they wouldn't really fit on the 737.

Not that this matters much since they ended the 757 in 2004.

We can't really know what would have happened but it is possible introducing a telescoping landing gear system to let the 757 MAX sit higher off the ground would have avoided all of these problems. The engines could have remained at their actual design location meaning no need for MCAS. The failure mode would be failure to retract landing gear if the telescoping system failed... rather than uncommanded pitch changes. A simpler failure mode with less risk.

None of this would have helped with Boeing's current problem (and a problem that plagued the 787 too): splitting off bits of the company and massive outsourcing to screw labor as hard as possible.

Folks don't seem to remember the 787 was a huge fiasco because Boeing outsourced so much of that aircraft and their vendors were unable to deliver parts on the required timelines, at the required quality, or in the required quantity. Boeing ended up having to buy a bunch of them to bring work back in-house to rescue the program.

Spirit Aerosystems was just the same strategy: outsource construction of the airframe (!!) by spinning off that department as a separate company so they could easily squeeze their now-vendor which would by design turn around and squeeze the employees. Kick them out of the Boeing retirement plan, cut benefits and pay, etc. Is anyone surprised Spirit filled positions with the cheapest bodies they could find? Think about the whole door fiasco: the ticket filed by Boeing QA was initially closed by the Spirit people basically doing nothing and hoping QA would just close the ticket without checking. That behavior didn't raise any alarms within Boeing indicating it was normal behavior by that point.