MCAS is needed to meet certification requirements. [0][1] Your understanding is common, but wrong, because the media commonly misunderstood what it was for. It's not a anti-stall system. It's not required to behave exactly like the old one. It is required that the stick forces monotonically increase until the plane stalls, which it does not without MCAS.

And it's required because they couldn't rip out all the old control system and replace it with a fly by wire system.

0: https://www.law.cornell.edu/cfr/text/14/25.173 1: https://www.law.cornell.edu/cfr/text/14/25.175