Ramban's argument explaining the Mishkan as a perpetuation of Har Sinai must be correct. So much of chumash deals extensively with the details of the Mishkan and sacrifices. Is it thinkable that so many mitzvot were simply an afterthought due to the sin of the Golden Calf?
On the other hand, Rashi's argument seems quite logical. After all, the striking similarities between the Mishkan and Chet Ha'egel cannot be overlooked.