Boeing would have preferred to have introduced a new narrow body aircraft, on their own schedule. But the new version of the 737 was announced before they got there, by American Airlines.
At that point it's either repudiate AA's announcement, harming relations with a customer worth at least 100 aircraft, or go build the thing. And if they build it, they're stuck with a bunch of the problems with the 737, like being unable to change how the controls work.
If they could sub out the control system from mechanical cables to fly by wire, they wouldn't have had any visible issues with the Max.
At that point it's either repudiate AA's announcement, harming relations with a customer worth at least 100 aircraft, or go build the thing. And if they build it, they're stuck with a bunch of the problems with the 737, like being unable to change how the controls work.
If they could sub out the control system from mechanical cables to fly by wire, they wouldn't have had any visible issues with the Max.