No one is "pretending" to know anything. There is a body of knowledge available from which to make predictions. Is it safe to say, based on the performances of rookie CBs coming into the league over the last decade, that mcCourty will perform worse than someone like Bodden (a proven, quality CB)? Very likely. If he does as well or better, I would consider this an abberation.
We have a pretty good body of work on Spikes abilities based on the preseason, and based on the fact that he displaced Guyton. We should be safe predicting he will at least perform as well as Guyton, considering that he beat him out over the course of many practices, game film analysis, and preseason work.
True, elite abilities are there from day one. It is raw, but you can see it. I think it is a myth that players abilities make some kind of drastic ability improvement during the course of 1-3 years. Can they improve? YES. But it is more likely incremental and not radical. The time is more a learning curve to digest the system.
For example, Maroney, Merriweather, Mayo, et al have not had some great talent burst their 2nd year. They were just better mentally, because they understood the system better.
Steroids are the only thing that changes this, IMO.