Q. According to the opinion of Rashi and the Sforno that the Mishkan was given only to redeem for the sin of the egel. Does that mean that without the egel we would be missing the largest single subject of the Torah; the Bet Hamikdash, all mitzvos of korbanot and kehuna?

A. Indeed, Rashi (beginning of Pekudey 38: 21 and Shir hashirim 1: 13) Medresh Rabba and Tanchuma (Terumah 8) and also Seforno (25: 9) all maintain that the Mishkan although mentioned before the chapter of the egel in Ki Tisa, was given to redeem the sin of the golden calf. According to Rashi, (31: 18) the parshios are presented out of chronological order.
However the Ramban (25:1 and 35:1), maintains that there is a seamless connection between Ma’amad Har Sinai and the building of the Mishkan. The Mishkan is a continuation of the revelation at Sinai and the kedusha oh Har Sinai rested on the Mishkan. This idea is central in the Ramban’s thought and appears a number of times in his writings. According to the Seforno, (see Seforno’s introduction to Bereshis and commentary on Shemos 20:21-23, 25:9 and 31:18, and Vayikra 11:2 and26:11-12).not only the Mishkan that was built in the wilderness, but the Beis Ha-Mikdash itself is a result of Yisrael’s weaknesses that brought about the cheit ha-egel. In his commentary Maskil Ledovid, R. David Pardo brings the view that Rashi also held that the concept of building the Mishkan was told to Moshe on Mount Sinai even before Chet Ha-Egel, and that Moshe was shown an image of the Mishkan at that time. He narrows the difference of opinion between Rashi and Ramban to a question as to whether the details of the command to build the Mishkan as presented in Parashas Terumah preceded Chet Ha-Egel (Ramban) or followed Chet Ha-Egel (Rashi).
Horav Shlomo Miller’s Shlit”a opinion is that even if you uphold the Midrashim and Rashi’s idea, the Mishkan and all the mitzvos that directly and indirectly depended on it, would have been granted if not by the egel’s sin, then by another reason, since they are part of the mitzvos of the Torah. This being similar to Am Yisroel descending to Mitzraim because of Yosef’s story. If that would not have happened, something else would have been the motive.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a.