What you fail to account for is the speed at which things can happen. No instructor can instantly take control when a student's plane is out of control. There is a constant evaluation going on in the instructor's mind of whether the student is in trouble, and if they can get themself out of it. There will always be a delay between when the student gets in trouble and when the instructor takes control. Due to the faster speeds of a warbird, significantly more distance will be covered during this delay. The odds are so stacked against learning with a warbird, buddy boxed or not, that it's not worth doing.

In addition, being an instructor and having a responsibility to protect the student's plane from the student's mistakes is challening enough with a trainer. Most are not willing to accept the additional risk, and stress, created by trying to teach with a warbird.