Aircraft with dangerous stall charactreistic is difficult to get certified. STS is a work around to help that. Same as it happened with B727 series. Boeing was able to get it through FAA(just like the max) but D P Davies refused to certify for CAA UK till a stick pusher was installed seven months later. MCAS is extended part of the STS. It was concealed to avoid expenditure and delays due recertification. If aircraft is flown well within the envelope no one may notice it's presence. Boeing had it in mind to upgrade the MCAS software in their own time but postponed it. Perhaps if a certain number of hours are flown without a problem then it may not even be required as it happened In case of B727. It kept flying in the US without the stick pusher. But whether 707 or 737 trim wheel all the time going crazy one way or the other is very noisy for sure.