I believe there is more than a high -hot- humid -runway length problem involved.
The 737s that crashed developed airplane problems while in flight and not in the process of landing or taking off. - Tom
The problem is that the aircraft is more vulnerable to stall than previous versions of the aircraft.
The problem of aerodynamic stall is worst while climbing out after takeoff. This problem is even worse at what is called “high density altitudes”, which is to say High/Hot locations, where the air is less dense.
Less dense air means that the aircraft does not produce as much lift at the same speed as it would at lower altitudes in denser air. This makes the aircraft more likely to stall.
The MCAS system, which has now been disabled, apparently, was designed to prevent a stall from developing by seizing control of the aircraft and pointing the nose down, so the aicraft would pick up speed and come out of stall.
Now Boeing is saying that the aicraft should not be flown into High/Hot airports. So it looks like they are solving the instability problem by just keeping the aircraft away from conditions that are likely to lead to stalls.