My understanding was that this system was installed in order for the new plane to behave exactly like the old one therefore not requiring costly additional pilot training.
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
However, the bottleneck is that there aren't very many sims worldwide, and most are owned by airlines that don't exactly have an incentive to get pilots other than their own trained.
The proposed fix is to change the software to admit to itself that it can fail, to limit the amount of control that MCAS can exert, and to train pilots to recognize failure modes and how, in the event of MCAS failure, manually handle the undesireable aerodynamics. Boeing can't simply disable MCAS and say that pilots have to always manually handle it; the rules specifically say the plane cannot behave in that manner.
[0] In some uncommon-but-not-impossible situations, pulling back on the control yoke and putting the plane closer to a stall is easier than pushing on the yoke and getting further from stalling. That isn't allowed for commercial aircraft.
https://www.nytimes.com/2019/06/01/business/boeing-737-max-c...
https://www.ecfr.gov/cgi-bin/text-idx?node=14:1.0.1.3.11#se1...
MCAS fixes this by detecting the condition and, in effect, pushing forward on a secondary control.
New type rating requires most training. Then there transition training between same aircraft family. These changes are probably just small updates that require least training.
Because Boeing and the FAA are talking out of both sides of their mouth. The planes aren't safe to fly without MCAS, MCAS is unreliable, so the solution is to disable MCAS whenever the plane determines it's best, even if it's going to be highly detrimental to safely operating. We still don't have ANY data on flying the planes safely without MCAS. We only have data on planes where MCAS fails, and those planes crashed.
I think you need to read up on MCAS a bit more.
The regulation is clear cut, and unambiguous in that regard. Furthermore, the crashes that occurred happened because a system that is only supposed to kick in at the extremes did so in non-extreme situations repeatedly due to GIGO (Garbage In, Garbage Out), and to disastrous effect.
I welcome you to look at the FDR telemetry curves for the two flights. The AoA measurement for one of them was 70-80 degrees if I recall, the other was 20ish degrees offset from where it should have been.