It's hard to say with Coach P because I don't think she has done a good job of keeping Duke a national powerhouse, but a lot of it can be blamed on bad luck with injuries/transfers.
She took over in 2008 after Duke lost Bales/Harding, they were going to struggle regardless and still earned a #3 seed and made the Sweet 16. Not great by Duke standards, but not awful for first year.
In 2009, they had a solid season earning a #1 seed and lost to Michigan State.
In 2010, another good year with a last minute choke job against Baylor in the Elite 8, the closest they've been to a Final Four. Finished about on par for what most expected.
2011, they get romped by UCONN. I'd say they underachieved this year with Thomas/Thomas as seniors and a great freshman class.
2012 they have a young but talented lineup. They caught a tough break being arguably the 5th best team in the nation, where Baylor/UCONN/Notre Dame/Stanford were clearly 1-4.
2013 they lose Gray and match up in the Elite 8 as a 2 loss team with Notre Dame...still a solid season, can't blame P for that season.
2014 they lose Gray and Jones and fizzle in the 2nd round
2015 they lose Jones to injury and Calhoun to a midseason transfer. Still put together some good wins but are starting to tank.
There have been valid reasons almost every season for Duke's lack of success, but Duke has yet to overachieve under Coach P, and have done nothing to prove they can hang with elite teams, as they are consistently throttled by Connecticut and I don't think have any top 5 wins. They are a perennial Sweet 16 caliber squad but coming from GG who made Duke into a power house and consistent title threat, Duke has definitely taken a step down and with this coaching staff there are no signs that Duke will be a title contender.[/Q
As an outsider, it seems really plausible to me that this is a snowball effect. Geno is very hard on his players sometimes, but constructively, with an important element of quid pro quo. If you are a highschool kid who wants to be an All American and win a National Championship, you know that his methods are proven. As that trust that the coach is doing what is best for you drops, so does your willingness to tough it out.
When McCallie is tough on her players, it doesn't seem constructive or productive and, frankly, at a distance at least it comes off as malicious (not letting players see their families because they lost, the infamous not-so-veiled tweets about players, etc.). As McCallie has now gone several years without meeting up to the expectations of fans, the program, or herself, it seems really easy to imagine that players are less willing to endure that. And, indeed, we're starting to see players who had the opportunity for a ton of playing time transfer away, something which is fairly rare in general.
Now that Duke is under performing even more-so this year, I can see the locker room dynamic getting even worse, and this can turn into a viscous circle which gets out of control really fast. For this reason, I think the (understandable) complaints about McCallie's Xs and Os are secondary to the fact that she doesn't seem to have the loyalty or trust of her players. It will be really telling if more players transfer at the end of this season.
which reminds me - while I have not heard the Calhoun transfer rationale, I can say that if I traveled to see my daughter play then was told I couldn't see her because they lost, I'd consider drastic action.