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.
"bunch of grad students"="patch MCAS to not auto-crash"
"really complicated"="redesign entire compute architecture to be multi-master and failure tolerant"
> “It’s really complicated,” John Hansman, an aeronautics and astronautics professor at the Massachusetts Institute of Technology who is not involved in the repair, said of revising aircraft software. “It totally makes sense why it’s taking longer.”
But then...
> While the fix became more complex and politically charged after the second accident -- the crash of an Ethiopian Airlines jet on March 10 -- the changes to MCAS remained self-contained and relatively simple. “I could have a bunch of graduate students and rewrite MCAS in a couple of days and be done,” Hansman said.
So is it "really complicated" or "relatively simple" to the point where "a bunch of graduate students [could] rewrite MCAS in a couple of days and be done"? It can't be both.