Exactly, using mechanical vanes for this sounds pretty stupid, given what can be done with acoustical signal processing. It must be a more difficult problem than it seems.
Not necessarily. It could be that after doing the safety analysis, the mechanical method is less likely to fail.
Remember that there are usually many ways to skin a cat. Often the one chosen hits the sweet spot of reliability, manufacturability, cost (both initial purchase and operational), maintainability (e.g., how easy is it to test/inspect), etc.
When you have decades of experience with something and it works well enough, there's generally no reason to improve it when your engineering resources can be focused on more pressing problems.
I was a team lead on a new system that was based on the IBM ISA bus in 2013. The reasons were simply that it worked well enough and we had tons of experience with it and could reuse a lot of custom-built hardware and code. The only downside (and reason we switched), was that it was getting harder to find blade PCs that still had an ISA bus and the system was expected to be in the field for 15+ years.
I was hardware lead on a system that was the 2013 revision to an existing ISA-bus blade PC architecture. We threw it all out and switched to an off-the-shelf DIN rail PC from Beckhoff with snap-on EtherCAT modules. It took a little extra development time but it was 100% worth it.
