There's much more reason to do a greenfield project in C++ than Rust - experienced C++ hiring is still considerably easier! Not everything has a purely technical motivator.
Perhaps, but you can take experienced devs with a background in other languages and expect them to write solid Rust code. You probably only need to hire 1 or 2 people who already know Rust.
If you can program C++ you'll pick upp Rust more quickly than any other convert. Unless it's a startup with a short runway where you might not have the luxury of a slower start, then I think it would probably pay of in both productivity, staff retention, ease of recruiting (later), and a lot of other parameters.