It is early to be making assumptions about the cause of this crash but it seems there is already good evidence of failures in the design/regulatory/certification process and that these need proper investigation.
I am not a pilot and although I design systems with safety aspects I am not in the aerospace areas. Despite this a key question is why a design change intended as a risk control measure seems to have introduced significant new risks.
On the face of it both the design/development/change control process seems to have failed but also the certification process in considering the impact of a change.
Given what MCAS does the risks of it failing to operate correctly do not need to have been properly consideer and controlled and although there is an element of hindsight it seems quite a stretch to have it vulnerable to a single point failure and with the assumptions that the pilots could control the risk of it failing under all reasonable circumstances and conditiosn without specific training.
There will be a focus on the specific cause of the accident but I would argue more worrying is what looks like a breakdown in the safety/regulatory process with respect to design modifications. Certainly for medical devices (my expertise) statistics suggest that modification of software is one of the largest causes of safety incidents and is therefore an area of focus for regulators.