Past his "Peak" is different than past his "Prime". To me, "Prime" infers a range of years whereas "Peak" implies the single best year. I see most of the disagreement here relating to one person arguing about prime and another arguing about peak.
There does not seem to be a compelling argument that Rondo is beyond his prime years. You could argue that the peak point of his career arc is behind him but so what. It is only about what he has left and I think we will see a few more years of Rondo playing pretty close to Rondo playing at his peak.
There is also a big difference in opinion regarding just how good Rondo is (or was) at this theoretical "peak". I feel that many Rondo advocates have a skewed opinion of just how good Rondo was (or is) because he got to play with the big 3 future HOFers. Rondo can play for the next few years at exactly the same level as the previous years and he is not going to appear to be the same Rondo. Rondo as the best player on a bad team will not be the same Rondo from the good teams.
When I see statements like "Rondo was the best player on such and such past Celtics team", I am really dismayed. Rondo excelled on those teams because there were other HOF players on the team to protect him from his flaws. Now on a bad team, these flaws are going to be more exposed and easier for other teams to exploit. When this happens, some will clamor "see, I told you he was past his prime", from others we will hear, "Rondo has lost his explosiveness and isn't the same Rondo".
Rondo could still improve and reach new "peaks" and extend his prime years just by learning to shoot better. This would more than offset any diminishment of athleticism, at least for a period of time. If that happens, people will say, "you were wrong Vermont Green, see how good Rondo is", but it doesn't change the past. The truth that I see is that to this point in his career, Rondo is a freakishly talented player who can't shoot and stubbornly tries too hard to get assists at the expense of good general ball movement.