Comment by karmelapple

Rewriting software is not only costly and subject to breakage, but for some of these systems requires an absolutely monumental FAA recertification process.

The cost of recertifying software shouldn't be the sole reason not to rewrite something, but I imagine you've been part of a rewrite where not quite everything worked as intended, even after having a lot of tests.

"The devil you know" very much applies to software that controls such life-critical functions and flying airplanes. If a pilot knows how to work around something, introducing something they may not know how to work around could be the difference between life and death.

Why did two 737-MAXes crash and the fleet grounded? New systems were introduced that pilots didn't know how to address - and seemingly could not workaround, even in spite of the engineers who designed them not wanting that outcome.

A rewrite, even with the most rigorous architectural and QA standards, is not a panacea.