> It seems those old Macs may still be in service in niche applications, most notably in audio processing chains at radio stations or recording studios.

Highly recommend looking at the tour of fat-boy slims recording studio for an example of this kind of airgapped system in active use for decades: https://youtu.be/qLjgXPDzeZo

(Though he uses an Atari ST, which is known for being a musical powerhouse)