I think theorising on Neuner's retirement is probably a blind alley at least for the foreseeable future. I think we may see something similar from Fourcade as well. Fourcade has also been outspoken with regard to anti-doping, bringing the subject up without being asked on more than one occasion. He's also said he doesn't see himself staying in the sport forever. For Neuner, she'd achieved pretty much everything she could from the sport, Olympic golds, World championships, almost single-handedly winning a relay for example, the World Cup overall, and the thrill of victory to her had become devalued to the point of being meaningless, while the expectations of her became ever greater. The only thing left for her was to go for Forsberg's records. I've no doubt that if she stayed in the sport she'd have smashed them, since at the age Neuner retired Tora Berger had never podiumed a World Cup race (!!!), but obviously stat-padding did not appeal to her (and I can imagine for Fourcade, with Bjørndalen's records being far in excess of Forsberg's, this will apply as well). Her current pregnancy may also point us in the direction of an answer. She isn't the only one to retire young in similar circumstances of course - Helena Ekholm retired at the same time, and the Swedish team has fallen off a cliff since!
If Neuner were to have retired fed up having won everything clean and not been able to get the team to clean up, there would have to be some dissonance there. For example, her former roommate and close friend Miriam Gössner remains on the team. Would she have been part of the team doping while Neuner was competing clean, or would she have been a clean athlete similarly marooned in a doping team (and similarly lightning quick but unreliable)? They have remained close friends since Neuner's retirement, so if doping was a big personal issue for Neuner I would find it difficult to believe that, if the two were not in agreement on the issue, her relationship with Gössner has never so much as faltered. And if Neuner were to have retired in frustration at the inability to get the team to clean up, it is notable to see the drop-off in German results last season, both women and men. While people like Greis have the excuse of age, Arnd Peiffer dropped off the face of the planet last year. Tina Bachmann also, although she has since found a glandular condition that explains her slow decline from the pinnacle of the sport to the third division. If the doping continues in Germany, then how come they haven't been able to replace all of the retirees, especially in respect of the women's team, which currently consists of a 36-year-old who was there long before the various retirees, a 33-year-old XC skier, a 26-year-old who's reliable and talented but has never podiumed a World Cup race, a 23-year-old who's returning from a career- and disability-threatening injury and two Juniors? Why have those successful juniors of the last few years - Juliane Döll, Caro Hennecke, Karo Horchler, Lisa Voigt, Nicole Wötzel, Carolin Leunig, Miriam Behringer, Maren Hammerschmidt - not been able to make the step up?
I think it's too simple an answer to too complex a question.