Other large software rewrites have unit tests. ATC has human operators and people are non-deterministic and non repeatable and you can't just run a few tests a

Other large software rewrites have unit tests. ATC has human operators and people are non-deterministic and non repeatable and you can't just run a few tests and conclude it's fine. The issues you're trying to stop may only occur once a month globally across all airports or something, and only be noticed once a year.