The reason I am saying most likely is that the Ethiopian pilots are reported to have followed the Boeing guidelines for disabling MCAS however as you stated, they left other aspects of the flight uncontrolled which complicated the situation to the point of fatality.
In fact, they DID NOT follow the Boeing procedure for disabling the MCAS. They did turn off the Stab Trim switches eventually BUT they re-engaged them later. The Boeing procedure states to leave them OFF.
You have hit the crux of the issue when you say "they left other aspects of the flight uncontrolled which complicated the situation to the point of fatality".
This is where the professional pilots in the industry part company with the Blame Boeing bunch. The Ethiopian crew left the power at 94% (takeoff), were climbing at 1500fpm while overspeeding the airframe with the stick full aft. They did disable the Stab Trim and tried to trim manually (while holding the stick full aft) but the aerodynamic load on the stab while exceeding Vmo would essentially prevent King Kong from being able to manually trim the stab.
That tale of the FDR is NOT the tale of a well trained experienced crew. You can say they were overwhelmed or confused; that's probably true. That's where training and experience come to the fore. When these two Captains had the last chance to break the accident chain...they failed in their duty.
The Runaway Stab Trim procedure really hasn't changed in the Boeing fleet from the B707 up to now. These two Captains supposedly had prior Boeing and specifically B737 experience. If so, they had undoubtedly _experienced_ the malfunction and procedure in simulators multiple times. If not, their training was deficient.
Further, these disasters begin with a simple AOA malfunction. This is another malfunction that they _should_ have seen in training in other aircraft. It doesn't matter which aircraft; AOA malfunctions are initially countered by using normal pitch and power to get to a safe altitude to work the problem in all aircraft. That did not happen in either Max accident.
As to MCAS activation, again I'll point out very basic airmanship. I cannot imagine that these two Captains had never heard or been exposed to the standard procedure, common to all aircraft everywhere, that when you move a secondary flight control (flaps) and the aircraft immediately acts in an unexpected, uncommanded manner, you IMMEDIATELY put that flight control back where it was. Thus, when both crews retracted the flaps and the nose pitched down unexpectedly and uncommanded, the flap lever should have immediately been returned to the previous position. MCAS does not work with flaps extended. Period. So had these Captains done what pilots have been told to do since secondary flight controls were invented, no MCAS, no crashes. Again, this is Airmanship 101. You don't need to know anything about MCAS to do this basic procedure. There are things you just have to know.
I could go on with the fatal flaws in crew performance. This is long enough, however. Clearly, as Puma (and others have pointed out repeatedly) the crew is always in a position to break the accident chain at some point. These two crews did not have what was necessary, be it training or experience or even just the ability to think and act under pressure. The mantra "maintain aircraft control, analyze the situation and take the proper action" was beyond their abilities on those two days.
Now, don't take all this to mean I feel Boeing is blameless. Boeing bears guilt, without doubt.
1) Boeing went to the well one time too many on the 737. Should have just bit the bullet and built a new aircraft to compete with the Neo.
2) Boeing had to be IDIOTS not to include detailed information on the MCAS system in the docs and training, to include the specific abnormal/emergency procedure.
3) AOA displays should have been standard on the PFD along with the disagree light. Charging extra for those on a modern aircraft is ridiculous. Doubt another gauge would have saved Lion/Ethiopian though. AF447 that went down in the South Atlantic didn't have an AOA gauge; they didn't ground the Airbus for that.
4) This whole debacle isn't over yet. It may well kill Boeing. The continual screwups with the KC-46 Tanker are also damaging the Boeing brand. Together, it makes for deep problems selling Boeing aircraft. With the Max and KC-46 they are looking like the gang that couldn't shoot straight. I place these problems primarily at the feet of management. (While McDonnell built great fighters, they built lousy transports. MD-11 as a prime example. Does the stretching the great DC-9 into the not so great MD-88 presage the stretching of the great 737 to no so great Max? The injection of McD managers into Boeing after the purchase of McD did not improve Boeings transport aircraft.)
Still, despite Boeing's mistakes, both aircraft were flyable. Put the flaps back out, no MCAS activation. Use the Stab Trim Cutout switches, no MCAS activation. Pitch/Power/Performance - no crashes. In the end, the pilots failed to break the accident chain. Neither crew was up to the challenge at that moment on that day.
