In his Running Formula, Daniels says that many people never reach their absolute limits, instead they only encounter their seasonal limits, which he claims to be dictated by their lifestyle at that given moment. Like, amount of rest, other stresses from life, nutrition, and (he doesn't says it) but I believe that peaking is also a factor (once you peak, you have reached your limit for that training season)
So, what I would want to know is, have someone here, really reached his/her absolute limits, like, doesn't matter, how much and smarter you trained, how much rest you took, how well you eaten, how much you lift and for how much seasons you countinued to do this, you simply didn't improve because each and every system in your body had reached it's pishyological limit, and how much people do you think ever face those limits?