Boeing dragged its feet on a 737 replacement. When Airbus upgraded the 32X series with the NEO, Boeing knew it needed to play catchup.
Boeing really needs a clean sheet re-design of the 737. The airframe debuted in 1968....and it was based on the older 707.
Boeing didn't have enough time to develop a whole new type. I'm pretty sure this will be the last major revision of the 737. Boeing needed a plane to compete against the A320 NEO series. I don't really find anything wrong with doing the Max. The 737 Max is lighter at equivalent passenger densities and has the advantage on shorter flights of about 500 miles or less. The Max is more efficient in climb. It's only on the longer flights where most of the time is spent cruising that the A320 has better seat mile costs.
Boeing is probably waiting for GE and CFM to come out with a reliable geared turbofan engine before committing to a totally new type based on technologies of the 787 and 797. It makes no sense to try to build a whole new type before the engines are mature. Any new type will benefit from the new technologies that come along in the next 10 to 15 years.
The problem with the 737 Max MCAS is being tied to only one sensor. If that sensor is faulty, the MCAS can command the nose of the plane to point nose down. Any 737 pilot who is not proficient enough with how to handle runaway trim can get into trouble really quickly. The MCAS needs to be easier to disengage. Other than MCAS, the 737 Max the same as the 737 NG.
What you FAIL to realize is that the 737 Max does not fly like a 737 NG on climb out.
Boeing knew this and used MCAS to cover the difference. The FAA approved it.
But what you FAIL to understand is that, at first, Boeing did not disclose the details of this system to pilots. That is what led to the letter from the pilots union.
When things are going wrong in the cockpit and the stick shaker is going while the plane is nosing down, 737 NG pilots will do what they do in a 737 NG simulator.
There are no 737 Max simulators....yet. They are being built because the pilots demand it and Boeing realizes it screwed up big time.
There is a reason why there is a CRIMINAL investigation into this rushed plane. Both Boeing and the FAA are complicit in the deaths of over 300 people.