First off, there is a great deal of proprietary information contained in FADEC software - how can you make it open source without revealing your trade secrets to your potential competitors? A lot of that information can't even be exported without special licenses (we needed to keep special agreements in place between Boeing and Rolls Royce just to allow us to talk with each other about the engine control s/w).
The flaw in MCAS was it's hazard classification (no worse than Major). Had it been properly identified as potentially Catastrophic, it never would have been implemented the way it was.
Or are you suggesting Boeing, Airbus, Pratt, GE, and Rolls provide China (and everyone else) with all the information needed to build state of the art aircraft?