https://en.wikipedia.org/wiki/John_Yudkin
Lest we not forget his arch-enemy Ancel Keys
Makes you wonder how ELF's brother (PE) and daughter (WASM) are doing.
However, Cat Stevens/Yusuf Islam did write "Father & Son" - which brings tears to my eyes... when I think of my son and my dad.
I would feel happy for you and try my best to make sure we got a good handover. Your problem is with the management and not your fellow developers.
>> If you were a dev manager or hired this developer, how would you feel about them leaving in the circumstances described?
I would try and tap into your experience before it leaves the company. The boat needs steadying - there may be some obvious low lying fruit.
Bad experiences are just as valuable as good experiences.