"best" engineers vs "senior" engineers is a valid point. I think a lot of my comments on this thread are of the mode of "best engineer" should also be thought of as "best engineer for this environment / culture." I tend to stay away from people who say things like "top talent" and "10x engineer" and (worst yet) "Unicorn." You can't really make a judgement about an engineer without describing the environment they'll live in.

And also... maybe more importantly... there's a saying (almost a joke) in the military: "You don't go to war with the forces you want, you go to war with the forces you have." (or maybe it was "people" instead of "forces" and no doubt the Air Force replaces "forces" with "very expensive weapon system manufactured by lowest-bid contractors by people who happen to live in districts represented by members of the congressional armed forces committee.") And that might be what we're getting at here.

While it would be great to get the absolute best engineer at day one, it's more likely you're going to get an engineer that requires a fair bit of training and in-the-trenches experience.