osborne
Boeing started working on a clean sheet single aisle in the early 2000's (I had friends working on it) - but when Airbus launched the A320 NEO it caught Boeing far from ready to proceed. Had Boeing committed to the clean sheet 737 replacement, it would have meant conceding the single aisle market to Airbus for about 10 years (not just longer to market, but years get the production rate up to ~50/month) - hence the decision to rush the MAX into production.
As for all the issues with the 787 - it's still outsold the A350 by a wide margin, with nearly 1,000 delivered.