Re: The reason that the Max series need MCAS
Having only two sensors is the real problem. If there's a failure, there's no way for the software to decide which is wrong and therefore work out the real angle of attack. The real answer is to add an extra sensor, like you say - or more likely two extra sensors for symmetry.
This is why it's a potential financial black hole for Boeing. Adding the extra sensors to existing planes, revising the software to support voting in case of a failure, and testing (properly this time) is an absolute shed-load of work. It would be realistically three months to get even the first grounded planes safely airborne.
Any other approach, though, risks the possibility of a third accident which would put Boeing into bankruptcy.