Mishkan as remedy for separation

לפי שיטת רש"י שהמשכן נצטוו לעשות אחר החטא.

According to the approach of Rashi, that they were commanded to build the Mishkan only after the sin [of the Golden Calf] —

The Sefas Emes builds on Rashi's view that the command to construct the Mishkan came after the cheit ha'egel, not before.

א"כ נראה שלוחות ראשונות היו מוכנים להיות נמצא בתוך בני ישראל בלי ארון ומשכן.

If so, it appears that the first Luchos (Tablets) were destined to exist among Bnei Yisrael without an Aron or a Mishkan.

Had there been no sin, the original Tablets would have dwelt among the people with no need for any physical vessel or sanctuary to house them.

שהיו מיוחדים בנ"י להיות בלי גשמיות.

For Bnei Yisrael were set apart to be without physicality (gashmiyus).

In their original, elevated state, Bnei Yisrael were so refined that they were meant to live on a plane beyond materiality.

וחז"ל רמזו במשכן בכל מקום שתלך קיטון אחד עשה לי ואדור ביניכם כו'.

And Chazal alluded regarding the Mishkan: "Wherever you go, make Me one small chamber and I will dwell among you," etc.

Chazal's image of a portable "small chamber" for Hashem implies that the Mishkan is a concession to a state in which Hashem's Presence needs a designated place — rather than dwelling freely among the people.

כנראה כי קודם החטא לא היו נפרדים כלל מהבורא ית' רק אח"כ שהי' קצת פירוד הי' העצה ע"י המשכן וכליו:

It appears that before the sin they were not separated at all from the Creator, may He be blessed; only afterward, when there was some degree of separation, the remedy (eitzah) came by means of the Mishkan and its vessels.

Before the cheit there was no gap at all between Bnei Yisrael and Hashem, so no Mishkan was needed. Once the sin introduced a measure of separation (pirud), the Mishkan and its vessels became the means to bridge that gap and restore Hashem's indwelling.

Summary: Following Rashi that the Mishkan was commanded only after the cheit ha'egel, the Sefas Emes explains that originally Bnei Yisrael were so spiritually refined — without gashmiyus and wholly unseparated from Hashem — that the first Luchos could dwell among them with no Aron or sanctuary at all. Only once the sin created a measure of separation did the Mishkan and its vessels become the necessary remedy to restore Hashem's Presence in their midst.