To better understand this "machloket" it might be helpful to conjecture what would have happened had the nation of Israel not sinned at with the Golden Calf. According to Exodus (23:20-33), God was prepared to send a Malach (angel) to help them conquer the land. The distance from Har Sinai to Kadesh Barnea was no more than an eleven-day journey (Deuteronomy 1:2). The first wave of conquest could have been completed in a very short time. They had survived six weeks in the desert since leaving Egypt without a Mikdash -- what would be so terrible if they waited another few weeks or months until 'Har Habayit' and Jerusalem were conquered? A proper permanent structure could have been constructed immediately, and there would have been no need for a temporary Mishkan. So it could have been according to Rashi.

Enter Chet Ha'egel: the spiritual level of Bnei Yisrael plummets and the 'shchinah' leaves the camp. No longer is there a guarantee that God will help fight their battles. The nation of Israel are no longer spiritually ready to conquer the land. The conquest could take many years, possibly generations. Therefore, a temporary Mikdash is required to help rehabilitate the nation's spiritual character. Nevertheless, the Torah intentionally records this parasha before Chet Ha'egel, to emphasize the thematic connection between Har Sinai and the Mikdash, in accordance with the fundamental importance of this mitzvah.

Now, Rashi's argument seems to be the most logical. Why doesn't the Ramban accept this explanation?