If the AoA was truly way too high then insane trim would be required to correct that. MCAS and similar systems came about in response to AF 447 where the pilots entered a high AoA situation and stalled the plane into the Atlantic.
It's true in the case of the 737 MAX where the pitch-up moment caused by the engines exceeds the elevator authority in a low-speed, high-thrust situation if the pitch trim is also set too high.
MCAS will evidently work with less maximum nose-down trim than the current version has, as the proposed update places a limit on it.
