This. Even if it's just an experimental project, at some point down the line if there's a possibility that it's going to have to integrate with the rest of the system - you've just introduced significant expectations for the existing and new developers on the team. While I don't think that rust has a huge learning curve, it's different enough that you can't just easily transition from a different high-level language.

I would never introduce a new language to an existing code base without an exceptionally good reason.

That's a catch 22 though. If you never introduce it no one will learn it?

The people that mattered were on board with the experiment though. It was a healthy enough environment that the project failing if it did wasn't going to be terrible for my career