Making things compact and focused helps. Of mixed-approach languages, I see Kotlin's approach to integrating FP as more successful.
I think it is a matter of choice of audience. If you want pure functional languages, there are plenty that will rock your boat (Haskell, OCaml ...).
But, to my mind, I see Rust as a somewhat "modernised" C/C++ with, like Python, the most interesting and accessible parts of FP backed in. It is a balance and it is fragile. So, as I said, I hope that Rust will keep that fine line.
All that said, it only is my opinion, I might be wrong and the wider audience might be ready for "harder" FP. But I doubt it.
With C++, I can't say the same is true. Some libraries try to alleviate that, of course.
The influence of "FP" on rust is really more like the influence of Haskell's type system. Rust is pretty far from a functional language but it does share a lot of type system features with Haskell: type classes, sum and product types, HM type inference