Theorists knew how to build certain features but did not know whether the resulting language would scale up to work in practice. To determine that, they would have had to do the experiment of trying to get mass adoption for a language. No-one did that experiment until Rust did.

And of course just implementing linear types and trying to get people to use the language probably would have failed. Borrow checking, compiler error message engineering, and various kinds of social engineering were probably essential to make Rust practical.