For what it’s worth I don’t think what you’re describing here is the same thing I meant by “finger guns” at all! What you’re describing looks to me like good incremental development.

With my “fingers guns mode” phrase, I really had in mind a blithe disregard for any of the real-world engineering problems and constraints that we knew we would hit (because they just inhered in what we knew the solution needed to do). You don’t have to solve those right up front, but you do need to (a) acknowledge that they exist and (b) have some notion of what your approach to them will be.

I’m extremely in favor of taking a small team of veterans (who are open to doing something novel!) on a new system, as you say—let them leverage all their expertise, while being careful to avoid Second System Syndrome, to build something by using all of the knowledge about those pitfalls/problems/etc. to avoid them. (The risk there is that you can end up overfitting on previous experience! That’s real!)