Even if monotonic improvements to near bug-free complex software were actually feasible, it might be difficult to persuade legal systems that they should be treated that way.

In real world regulatory environments, Boeing's patch to its MCAS system - which was tractable and trivially simple compared with teaching a software program how to react safely to certain types of human behaviour whilst leaving behaviour in other situations unchanged - is undergoing months of testing and the fleet has been grounded for over a year.

I disagree, the MCAS system was a poor attempt at fixing a hardware design flaw with software. Boeing tried to slap larger engines on an airframe that was not designed for those engines to save cost. This made the aircraft more more prone to stalls, requiring the MCAS system. Boeing convinced the FAA not only that the MCAS system was sufficient for preventing stalls but also that pilots didn't need retraining. It's likely that MCAS is not sufficient and that's actually what is taking so long.

Obviously MCAS was not a success, but the passengers didn't die because of the hardware's propensity to stall, but because the software incorrectly handled an edge case it hadn't encountered in testing by fatally altering the aircraft's course. Needless to say, the risks of that happening when software takes 'everything going on the roads' as input and not just 'certain angle of attack parameters' are higher, not substantially lower as they would have to be for self-driving car manufacturers to be unconcerned about regulatory intervention or consequences.

The FAA always takes its sweet time making decisions, mostly for good reasons anybody regulating mass market autonomous road vehicles ought to follow. But what's spooked them here isn't software between a pilot and the aircraft's control surfaces altering the piloting experience (that's been around for a while), but the software making a decision and the pilots being unable to effectively override it...

@notahacker was not talking about the original MCAS, but about the patch to it -- the modifications now under development to fix the problems.

Right, but the patch is likely taking a long time because they're trying to fix an inherently unstable design with software.

This is possible in aviation but is typically only done fighter jets where you have vectored thrust, massive control surfaces, and an ejection seat if things go wrong.

If your goal is safety, software-based stability is a very poor design. I think there's a decent chance the MAX won't be re-certified without physical design changes.