The plane was designed with dual AoA sensors from the beginning.

Boeing management made a decision to sell dual sensors as an option package, cutting the base model down to one single AoA sensor per MCAS. (there are two MCAS systems to provide redundancy)

It was already built adequately, the decision to cut corners on safety to try and leverage a small profit was the problem.

Dual AoA sensors as part of the standard option package and providing adequate training to pilots would have prevented all of this.