Singular Dibros sustain Torah through one Jew

למה נכתב לשון יחיד ליתן פתחון פה למשה במעשה העגל כו'.

Why were [the Aseres HaDibros] written in the singular? To give Moshe an opening to plead at the incident of the Eigel (the Golden Calf)…

The Sefas Emes cites Chazal's question on why the Ten Commandments are phrased in the singular ("Anochi… your God," not "your God [plural]"). Their answer: so that Moshe Rabbeinu would have grounds to defend Bnei Yisrael after the cheit ha'Eigel.

נראה לפרש שניתנו באופן שיוכלו הדברות להתקיים אף בדור חייב שלא יהיו רוב ישראל בטוב רק יחיד אחד יכול ג"כ לקבל הדברות אלו ובכחו מתקיים התורה בישראל.

It appears one may explain that they were given in such a way that the commandments could be upheld even in a guilty generation — that even if the majority of Bnei Yisrael are not good, a single individual can also receive these commandments, and through his power the Torah endures within Bnei Yisrael.

The singular wording embeds a safeguard: the Dibros are addressed to each individual, so that even in a fallen generation where most have strayed, one solitary Jew can still fully receive them. The merit of that lone individual keeps the Torah alive among the entire nation.

ובל"ז לא הי' תקומה לשונאי ישראל במעשה העגל שלא הי' מתקיים אנכי רק על מרע"ה.

And without this, there would have been no standing for "the enemies of Israel" [a euphemism for Bnei Yisrael] after the incident of the Eigel, for "Anochi" would have been fulfilled only through Moshe Rabbeinu.

Had the Dibros been addressed only to the collective, then after the Eigel — when the nation as a whole faltered — there would have been no way for them to survive; "Anochi Hashem" would have remained upheld solely by Moshe. The singular phrasing is precisely what allowed even a depleted nation to retain its claim to the Torah.

וזה הפתחון פה לומר שעדיין התורה מתקיימת בישראל כנ"ל:

And this is the "opening" — to say that the Torah still endures within Bnei Yisrael, as above.

This was Moshe's defense: because the Dibros were given to each individual, even after the Eigel the Torah was still being upheld within Klal Yisrael through those who remained faithful. That argument is the "pischon peh," the opening Moshe needed to win their forgiveness.

Summary: The Aseres HaDibros were written in the singular so that each Jew receives them individually. This ensured that even in a guilty generation — and even after the cheit ha'Eigel — a single faithful individual could uphold the Torah and keep it alive within all of Bnei Yisrael. That very feature gave Moshe Rabbeinu his "opening" to plead that the Torah still endures among the people.