MCAS isn't an anti stall system. It's a regulatory compliance system. Specifically, it's to comply with a regulation requiring that the force on the stick increase with increasing angle of attack.
The situation that the regulation addresses should never occur on a fare carrying flight. It's at the edge of the flight envelope, and a pilot who deliberately flies at the edge of the envelope with fares in the back should be prosecuted.
That said, sometimes things break, or weather causes things to go wrong. This may result in inadvertently flying near the edge of the envelope.
To a rough approximation, 0 out of a 1000 flights will meet the requirements for MCAS to activate.
We do know exactly the conditions required for MCAS activation. And we do know those conditions should never be encountered during a fare carrying flight. Any pilot who deliberately puts their aircraft into conditions that result in MCAS activation should be prosecuted.
Pilots should never come close to stalling during fare carrying flight. Therefore the MCAS should never activate during routine flight.
We do not know the conditions, we know the conditions Boeing said, and we know that Boeing said it's rare. The data from flights before grounding could answer this question, but it's nowhere to be found in the report. It's not even discussed.
The situation that the regulation addresses should never occur on a fare carrying flight. It's at the edge of the flight envelope, and a pilot who deliberately flies at the edge of the envelope with fares in the back should be prosecuted.
That said, sometimes things break, or weather causes things to go wrong. This may result in inadvertently flying near the edge of the envelope.
To a rough approximation, 0 out of a 1000 flights will meet the requirements for MCAS to activate.