The Type Certicate Data Sheet for the Boeing 737 is A16WE. It covers every model of 737 made and was issued in 1967. The FAA also only issues one type rating that covers every model ever made. Every possible effort was made to avoid the requirement to recertify changes. This combined with a corporate culture of “do it cheap and fast” instead of “do it right”, made this debacle inevitable.
Speaking of 2020 hindsight, how about the 747 program. The CEO bet the company on what was a transformational engineering aspiration. 777 and the 787 permanently changed the airliner landscape, and yes I am fully aware of the 787 problems but it was not a lack of vision that screwed up this program it was a failure of execution when the bean counters started to displace the engineers.
Sadly the 737 program was a failure of both imagination and execution. A clean sheet design was off the table almost immediately because the only decision criteria was what would goose the stock price this quarter.
The true irony is if they had gone with a clean sheet design the entry into service engine woes would have significantly blunted Airbus first mover advantage and going forward they would be offering a brand new airframe against an almost 30 year old Airbus 320 competitor, but that would have required the kind of vision in the C suite you do not see much of these days.....