Simply put, most people do not have the mental power to be able to truly max themselves out. Right before you truly max out, that tipping point, is when you reach your genetic potential and set the best PR ever. After you set it, your body will be toasted, burned out, and you'll quit the sport or never get back to where you were. Because you maxed out. If that makes sense, my English is not the greatest
On the other hand if we throw micro dosing Test, EPO, caffeine, and painkillers into the equation I think very well sub 14 is doable for serious runners