What happened with G-d's promise?

Question

In Bereshit 15:18 G-d makes a covenant with Abraham in which He says that He has given certain land to Abraham's descendants.

בַּיּוֹם הַהוּא כָּרַת יְהֹוָה אֶת אַבְרָם בְּרִית לֵאמֹר לְזַרְעֲךָ נָתַתִּי אֶת הָאָרֶץ הַזֹּאת מִנְּהַר מִצְרַיִם עַד הַנָּהָר הַגָּדֹל נְהַר פְּרָת:
On that day, the Lord formed a covenant with Abram, saying, "To your seed I have given this land, from the river of Egypt until the great river, the Euphrates river.

At first, one could think that this refers to Eretz Israel after the exile from Egypt. But here G-d described a territory that would be drastically bigger than any jewish land/kingdom there has been.

My interpretation was that this is referring to the times of the Messiah. Based on the facts that: 1) The land of Israel will encompass three new cities when the Messiah arrives. 2) A bad decree can change, while a good decree will never change.*

*I don't have sources for my last two statements. I have either read them on the site or heard them from elsewhere.

Answer 1

The Ohr HaChaim learns like you do - and more - that this verse is one of the proofs that the Torah is predicting the future.

Something to the effect of:
"It's perplexing how they are incapable of believing in a great future! It's an explicit verse! Abraham was promised [this large piece of land] - the land belonging to the 10 nations enumerated [after this verse] and yet we never were given more than 7."

הנה יש לתמוה איך טח מראות עיניהם של הממאנים להאמין בגדולתינו ומעלתינו העתידה לבא, כי מקרא מלא דבר הכתוב שכרת ה' ברית עם אברהם אבינו עליו השלום לתת לזרעו עשרה אומות ועד עתה לא מצינו שנתן ה' לישראל אלא ז' אומות ונשארו ג' אומות ואיך יאמרו העצמות היבשות אבדה תקות נחלתינו ושבועת ה' מנגדתם

Answer 2

Rav Eliezer Melamed, in his work Pninei Halacha (העם והארץ ג:יד), quoting כפתור ופרח פרק י And הרב אברהם אזולאי (חסד לאברהם מעין ג, נהר ז describes those borders as the borders in which the mitzvah of settling the land applies (not for mitzvot tluyot baaretz).

He says the mitzvah to live there, is based on the borders promised to the avos, and that the shechina is not fully in Israel until Jews live in those borders.

He does go on to explain that nevertheless there is a certain level of greater kedusha to the west bank of the Jordan than other areas of Israel.