The Mikdash endures only through avodah

ובמדרש בפסוק אין זה כי אם בית אלקים שראה ביהמ"ק בנוי וחרב כו'.

And in the Midrash on the pasuk "This is none other than the house of God" (Bereishis 28:17) — that he saw the Bais Hamikdash built and destroyed.

When Yaakov Avinu awoke from his dream and declared the place to be the "house of God," the Midrash teaches that he was given a vision of the Bais Hamikdash both in its built glory and in its churban (destruction).

כי פי' אין זה כי אם בית אלקים הוא שביטל הכל לעבודת השי"ת וכ' הארץ אשר אתה שוכב דרשו חז"ל שקיפל כל ארץ ישראל תחתיו והם הד' אמות שכולל כל העולם כמ"ש ה' אלקיך דורש אותה ובשכר דרישה זו כולם נדרשים.

For the meaning of "This is none other than the house of God" is that he nullified everything to the avodah of Hashem; and it is written, "the land upon which you lie" — Chazal expounded that He folded all of Eretz Yisrael beneath him, and these are the four amos (cubits) that contain the whole world, as it is written, "Hashem your God seeks it out" (Devarim 11:12), and in the merit of this seeking, all are sought out.

"Ein zeh ki im bais Elokim" expresses Yaakov's bittul — he subordinated everything to the service of Hashem. Chazal say all of Eretz Yisrael was folded beneath the sleeping Yaakov; those four amos are a concentrated point containing the entire world. Because Hashem "seeks out" (dorush) that land, and Yaakov made himself a vessel for that seeking, through it the whole world is drawn into Hashem's care.

והכל ע"י מקום ביהמ"ק שכולל הכל וזה המקום נתן השי"ת ליעקב ויעקב ביטל המקום להשי"ת ואמר אין זה כו' שאין למקום זה קיום רק בעבודת אלקים.

And all of this is through the place of the Bais Hamikdash, which contains everything; and this place Hashem gave to Yaakov, and Yaakov nullified the place to Hashem and said "This is none other…" — that this place has no existence except through the avodah of God.

The site of the Mikdash is the all-inclusive point of the world. Hashem entrusted it to Yaakov, and Yaakov responded with bittul, devoting the place entirely to Hashem. Its whole reality and permanence depend solely on its serving as the locus of avodas Hashem.

ולכן כשחטאו בנ"י נחרב המקדש וכל זה הי' בכלל בחירת יעקב אע"ה כנ"ל:

Therefore, when Bnei Yisrael sinned, the Mikdash was destroyed; and all of this was included in the choice of Yaakov our father, peace be upon him, as above.

Since the place exists only through avodah, when Bnei Yisrael fell from that avodah through sin, the Mikdash necessarily fell as well — its standing was always conditional. Yaakov's vision of both the built and destroyed Temple, and his bittul of the place to Hashem, already contained this truth: the place lives or falls with the people's service of Hashem.

Summary: Yaakov's declaration "This is none other than the house of God" was an act of bittul, devoting the place of the Bais Hamikdash entirely to the avodah of Hashem. That site is the all-inclusive point of the world, and Hashem "seeks it out" so that through it the whole world is sustained. But precisely because its existence rests on avodah alone, when Bnei Yisrael sinned the Mikdash was destroyed — a reality already foreseen in Yaakov's vision of the Temple built and ruined.