The reason this discussion never goes anywhere is because the conversation is usually poorly argued from the outset. Just like in this thread. If you want to have a good discussion about Duke's big men (past, present, and future), it is important to approach the issue logically, rather than just jumping to a conclusion that there is a problem with the coaching and, specifically, that the problem with the coaching is that the coaches were guards or are too short.
the things to discuss would be:
1) Recruiting: Is Duke getting the right talent at PF and C positions? How does our recruiting success compare to other top programs? To the extent it is knowable, why have the recruits Duke has missed on chosen to go elsewhere?
2) Coaching / player development: Have Duke players developed at an appropriate rate compared to what is typical for players at their level? If not, what is the cause of the difference? The height of the coaches may be one possible reason to consider, but is certainly not the most obvious thing to look at
3) Style of play: Does the coaching staff favor a style of play that reduces the role of the bigs on offense? Or is that not true?
4) Results in college: How does Duke big performance, over time, compare to other programs?
5) Results in NBA: How do Duke bigs do in getting drafted and performing in the NBA vs. others?
These things are all interrelated, of course, but I think worthy of an interesting discussion which is impossible to have when you start from "our coaches must be too small"