yeah, I think you're right -- since you don't normally need the MCAS, you could probably get away w/ 2 AoA sensors and just disengage MCAS when they don't agree. you'd only get into trouble when you need MCAS and the AoA sensors are busted, which is a (rare) * (rare) event, so probably low enough odds to tolerate (i.e., probably never over lifetime of plane).

(just speculation of course.)