Chesed prevailing for the world's endurance

ברש"י יכול חותמין בכולם ת"ל והיה ברכה בך חותמין וכו'

In Rashi: one might think we seal [the blessing of the Shemoneh Esrei] with all of them — therefore the pasuk teaches "and you shall be a blessing," [meaning] we seal with you [Avraham] etc.

Rashi explains that although the Avos are all mentioned, the first brachah of Shemoneh Esrei concludes "Magen Avraham" — sealed with Avraham alone — based on "and you shall be a blessing."

וכן איתא שאמר אברהם אבינו ע"ה שמא יבוא אחד ויסגל מצות וידחה בריתו כו'.

And likewise it is brought that Avraham Avinu said: perhaps someone will come and amass mitzvos and thereby displace his (Avraham's) covenant etc.

The Midrash relates Avraham's concern that a future tzaddik might accumulate so many mitzvos as to override the bris and special standing granted to Avraham.

ותמוה כמ"ש במ"א בשם מו"ז ז"ל.

And this is puzzling, as discussed elsewhere in the name of my grandfather (Moreinu Zekeini), of blessed memory.

The Sefas Emes notes that this idea is difficult, citing a discussion he raised elsewhere in the name of his grandfather, the Chiddushei HaRim.

ונראה לפרש כפשוטו כי ידע אאע"ה כי יצחק ויעקב יבחרו להם כ"א דרך בפ"ע.

And it seems to explain it simply: that Avraham Avinu knew that Yitzchak and Yaakov would each choose for himself a separate path.

Avraham foresaw that his offspring Yitzchak and Yaakov would each develop his own distinct middah and avenue of avodah.

והעולם חסד יבנה וכיון שהוא היה המתחיל ביקש שיגבר מדת החסד על כל המדות כדי שיהיה להעולם קיום והבטיח לו השי"ת זאת.

And "the world is built on chesed," and since he was the one who began, he requested that the middah of chesed should prevail over all the other middos, so that the world should have endurance — and Hashem promised him this.

Because the world stands on chesed, and Avraham was the pioneer who embodied it, he asked that his trait of chesed dominate all the others so the world could endure — and Hashem granted it.

ולכן נאמר חסדי ה' כי לא תמנו:

And therefore it is said: "The kindnesses of Hashem, for they are never exhausted."

The pasuk (Eichah 3:22) confirms that Hashem's chesed never ceases — the enduring dominance of chesed that Avraham secured for the world.

Summary: Although Avraham worried a future tzaddik might "outdo" his covenant, the deeper point is that Avraham, as the pioneer of chesed — the middah on which the world is built — asked that chesed prevail over all other traits to give the world endurance; Hashem promised this, which is why the brachah is sealed "Magen Avraham" and why "Hashem's kindnesses never cease."