It's actually a bit worse than that. American Airlines actually announced the "new" 737 variant before Boeing was even thinking of it[0]. At which point Boeing needed to figure out how to make the next 737 happen.
Boeing was all in on building a clean-sheet replacement for the 737 up to like 2011 until American said basically "we're not buying any new planes unless they're 737s and we'll re-standardize on Airbus". Boeing still shouldn't have built it (you don't need to be an aerospace engineer to see where the issues were going to be) but talk about being put into a "coffin corner" (ha ha).
[0] http://news.aa.com/news/news-details/2011/AMR-Corporation-An...