However, modern AAA game dev is largely about customising and upgrading large existing game engines, which are all C++ based. Rust could be used for them, but isn't, and the extra features Rust brings are not compelling enough to rewrite huge amounts of working code and suffer (re)training costs.

This is a hole many new languages find themselves in. JetBrains have come up with a very clever solution for their Java/Scala competitor Kotlin: the language was carefully designed from the ground up with perfect Java interop in mind (Java/Kotlin call into each other seamlessly, the IDE can refactor across languages, Kotlin uses the same standard library etc). And quite amazingly, the IDE can automatically rewrite Java into Kotlin. This isn't a party trick, it works very well.

I think if Rust wants/wanted to be a serious competitor to or upgrade path from C++ it'd have needed to take at least some level of C++ interop into account from the start such that Rust and C++ could be intermixed freely. AFAIK though, the best you get is a typical C export/import layer which is too trivial to be compelling.

Of course, it's a lot harder than connecting with Java because of the way C++ stuff is defined using header files. But at least basic interop could have been done.