Fair point, although the AoA sensor wasn't just used for MCAS, it was also used to feed the AFCS.
There's actually also a bunch of logic Boeing employed to tell you if you think you have a problem with a single sensor - but as we've clearly seen, those tools can sometimes be flawed. The cost of three sensors, versus one or two, on an aeroplane as big and complex as a B737 is trivial and should have been done.
G