I agree. It is a lot of money, but that's the hope from paying engineers well: to make the chances of very expensive mistakes unlikely.

One thing I did think about was how this could have been architected without sufficient reference to costs, which might have been a process or structure improvement.

Right - if your engineering organization ships designs that are bad economically, the solution is to introduce a culture of predicting costs before committing to a design, and processes to help enforce that culture.

Add "expected budget, double-checked by at least one other principal engineer" to the project checklist.

Have the person most responsive for the $8m "mistake" be the person to drive that cultural change, since they now have the most credibility for why it's a useful step!