This argument can apply to every application scene of Rust: no need to learn a new language. Yet the fact is that the Rust community is growing rapidly, from audio to embedded devices. I think the key is how much you can get from the investment, and I totally understand if people want to stick to the ecosystem they are already familiar with, a completely different story compared with a beginner.
Also, I don't think the JUCE "rules in the audio industry". First, the most typical use case for JUCE is to write VST plugins. Second, the company ROLI was trying to develop another language called SOUL. Although its progress seems to stop now, it somehow shows that they are trying to provide something different from C++.
Which the author did quite alright, minus the issue I discussed on separate comment.
You will have better luck moving the audio industry from bad C practices into modern C++, than making most learn a complete new language.
JUCE rules in the audio industry for example.