BPA is so frequently not that clear. Even this year at 14, if we're looking at MBN, Eiserman, Solberg, Jiricek... Who's BPA? there's no consensus on these players, especially not for who they're going to be in 6 years and for how long. There's an outcome and a risk profile for each. Eiserman has the most elite single skill and has the highest draft profile but has fallen the most. Jiricek has pedigree but a risky injury profile. MBN may be closest to ready, but what does he have that's elite? Solberg is the massive riser who just held his own in an NHL-level tournament as an 18 year old and maybe his ceiling is super high but only in 7 years with risk.
Then there's the fact that D take longer to develop but they're drafted at the same time, which basically means that you're more likely to have a near-NHL ready "BPA" at forward in the first round. But does the player fit with what your team needs in 5 years, what's their peak, etc? I'd love to hear from actual professional front office people if they even discuss the concept of BPA.