Unfortunately we get the worst of all worlds usually. My background is largely with these kinds of legacy systems (across many industries), I've dealt with the mainframes that airlines and flight GDS like Sabre and Apollo still run on to this day... It's still riddled with bugs in stuff that can't really be modified for reasons, it's just that we figure we know most of them by now and none of them should bring a plane down or anything. There are some planes in the air with archaic interfaces where the operators just need to know not to do certain things too close together with the hardware controls because it might cause inconsistent states that lead to problems - though nothing that should bring down an airplane, but everyone's always thought that and then Air Disasters and shit still have new material...