The impression I got from the article is that they are bringing all (safety-critical?) control-system software up to 21st-century standards of redundancy. It w

The impression I got from the article is that they are bringing all (safety-critical?) control-system software up to 21st-century standards of redundancy. It was not clear to me whether it would be possible, in theory, to fix MCAS alone, and if so, who decided that it was necessary to go beyond that - Boeing? The FAA? Other countries' regulators? The airlines? None of these entities want another round of "you didn't fix it right", though their perception of risk and tolerance of it may vary. AFAIK from this article, none of the earlier generations of 737s will be so modified, even though their systems are presumably very similar, in all non-MCAS aspects, to what the MAX first flew with. That's quite arguably a rational choice, once you recognize that there is a cost - benefit tradeoff even in air-travel safety. This is a sort of technical debt situation. Ironically, if Boeing had not tried so hard to game the regulations, MAXes could be flying today with less fault-tolerance than they will have when they return.