This doesn't actually address Liat's question, though it is a common interpretation of the threat... Offer as an alternative explanation, but I THINK....
כי ביום אכלך ממנו מות תמות. כלו' דע כי סופך למות. מות תמות שלא תנצל מן המיתה לעת קץ, ולא אמר שבאותו יום ימות:
For on the day you eat from it, you will certainly die: Meaning to say, know that your end will be to die. You will certainly die, such that you will not be saved from death at the time of [your] end. But He did not say that he would die on that day.
This actually answers Liat's question of why the punishment isn't as severe.
כי שמעת לקול אשתך וגו׳. לא החל לומר כי אכלת מן העץ וגו׳ אלא בשביל שלא הענישו כמו שאמר כי ביום אכלך ממנו מות תמות. ע״כ ביאר לו יהוה שאם היה אוכל בשאט נפש ודאי היה ראוי להענש מיד. וכדרך הכתוב במיתת נו״א בקרבתם לפני יהוה וימותו שקרבתם לפני יהוה גרם להם שמתו מיד על חטאתם שהקריבו אש זרה. ויבואר שם. א״כ באדם נמי שהיה אז עומד וקרוב לפני יהוה אם היה חוטא במזיד היה ראוי שימות מיד. לכן אמר לו יהוה כי שמעת לקול אשתך ואין זה אלא שוגג. אין עונשך רק המוכרח לפי מעמדך כעת:
Because you listened to your wife, etc.: He did not begin [by] saying, "Because you ate from the tree, etc." Rather, because He did not punish him like He said - "for on the day you eat from it, you will surely die" (Genesis 2:17) - hence God explained to him that if he had eaten contemptuously, it certainly would have been fit to punish him immediately. And this is like the way of Scripture with Nadav and Avihu, "when they came close before the Lord and died" (Leviticus 16:1). Such that it was their coming close before the Lord that caused them to die immediately for their sin, since they brought a strange fire; and it is explained there. If so, also with Adam who was then standing there close in front of the Lord - if he had sinned volitionally, it would have been fit that he would die immediately. Hence God said to him, "Because you listened to your wife, etc." - and that is only inadvertent - your punishment is only that which is required according to your status now.
Kayin maybe did such a great job convincing G-d to be more lenient.
גדול עוני מנשוא עונש עוני שהטלת עלי לא אוכל לסבול שהרי גרשת אותי וכו׳ עוני כמו ויגדל עון בת עמי מחטאת סדום אם יקרך עון בדבר הזה. ד״א גדול עוני מנשוא בתמיה הן גרשת אותי ע״י שלא תוסיף הארץ לתת כחה לי, אבי חטא לך וגרשתו מגן עדן ונתכפר לו ואותי שגרשת להיות נע ונד היה לך לכפר לי.
גדול עוני מנשוא, “my sin is too great to bear.” Kayin refers to the punishment decreed as a result of his sin. It is more than he thinks he can bear. Having been expelled from having a permanent home on earth is already enough, in his opinion. We find a similar reaction in Lamentations 4,6 where Jeremiah describes the punishment of the Jewish people in similar terms, i.e. ויגדל עון בת עמי מחטאת סדום, “the guilt (and punishment) of my people exceeded that of the people of Sodom;” according to Ibn Ezra, Samuel I 28,10 is also to be understood in that sense. According to another exegesis, Kayin’s exclamation is to be understood as a query? “Is my sin really too great to be forgiven? You have already expelled me from my home etc., and made it impossible for me to plant roots, as the soil will not respond to my labour; my father has been expelled from Gan Eden for his sin, but You have seen fit to let him atone for it. Why do You not give me chance also to atone for my sin?” [Adam is not even on record as admitting having sinned. Ed.]
At the end is the main point: Kayin's argument to G-d was that man is weak and needs the amazing powerful G-d to protect him. His argument was expressing humility and recognizing G-d's greatness. "I'm weak and can't protect myself" and "you're all-powerful."
גָּדוֹל עֲוֹנִי מִנְּשׂוֹא בִּתְמִיָּה, אַתָּה טוֹעֵן עֶלְיוֹנִים וְתַחְתּוֹנִים, וַעֲוֹנִי אִי אֶפְשָׁר לִטְעֹן, לְשׁוֹן רַשִׁ"י (רש"י על בראשית ד':י"ג) מִבְּרֵאשִׁית רַבָּה (בראשית רבה כ"ב:י"א). וְהַנָּכוֹן בַּפְּשָׁט שֶׁהוּא וִדּוּי, אָמַר אֱמֶת כִּי עֲוֹנִי גָּדוֹל מִלִּסְלֹחַ, וְצַדִּיק אַתָּה ה' וְיָשָׁר מִשְׁפָּטֶיךָ, אַף עַל פִּי שֶׁעָנַשְׁתָּ אוֹתִי הַרְבֵּה מְאֹד וְהִנֵּה גֵּרַשְׁתָּ אֹתִי הַיּוֹם מֵעַל פְּנֵי הָאֲדָמָה, כִּי בִּהְיוֹתִי נָע וָנָד וְלֹא אוּכַל לַעֲמֹד בְּמָקוֹם אֶחָד, הִנֵּה אָנֹכִי מְגֹרָשׁ מִן הָאֲדָמָה וְאֵין מָקוֹם לִמְנוּחָתִי, וּמִפָּנֶיךָ אֶסָּתֵר כִּי לֹא אוּכַל לַעֲמֹד לְפָנֶיךָ לְהִתְפַּלֵּל אוֹ לְהַקְרִיב קָרְבָּן וּמִנְחָה, כִּי בֹּשְׁתִּי וְגַם נִכְלַמְתִּי כִּי נָשָׂאתִי חֶרְפַּת נְעוּרָי, אֲבָל מָה אֶעֱשֶׂה כִּי כָל מוֹצְאִי יַהַרְגֵנִי, וְאַתָּה בְּחַסְדְּךָ הָרַבִּים לֹא חִיַּבְתָּ אוֹתִי מִיתָה. וְהָעִנְיָן שֶׁאָמַר לְפָנָיו הִנֵּה חֶטְאִי גָּדוֹל וְהִרְבֵּיתָ עָלַי עֹנֶשׁ, אֲבָל שָׁמְרֵנִי שֶׁלֹּא אֵעָנֵשׁ יוֹתֵר מִמָּה שֶׁחִיַּבְתָּ אוֹתִי, כִּי בַּעֲבוּר שֶׁאֶהְיֶה נָע וָנָד וְלֹא אֶבְנֶה לִי בַּיִת וּגְדֵרוֹת בְּשׁוּם מָקוֹם, יַהַרְגוּנִי הַחַיּוֹת, כִּי סָר צִלְּךָ מֵעָלַי. הוֹדָה כִּי הָאָדָם אֵינֶנּוּ נִשְׂגָּב וְנִמְלָט בְּכֹחוֹ, רַק בִּשְׁמִירַת עֶלְיוֹן עָלָיו:
MY SIN IS GREATER THAN I CAN BEAR. This is in the form of a question: “You bear the worlds above and below, and is it impossible for You to bear my sin?” Thus the words of Rashi quoting Bereshith Rabbah.The correct plain interpretation is that it is a confession. Cain said: “It is true that my sin is too great to be forgiven, and Thou art righteous, O Eternal, and upright are Thy judgments even though You have punished me exceedingly. And now behold, Thou hast driven me out this day from the face of the land for in being a fugitive and a wanderer unable to stay in one place, behold, I am driven from the land and there is no place where I can find rest. And from Thy face shall I be hid whereas I will not be able to stand before You to pray or bring a sacrifice and meal-offering for I was ashamed, yea, even confounded, because I did bear the reproach of my youth. But what shall I do? Whosoever findeth me shall slay me, and You in Your manifold loving-kindness did not decree death upon me.” The sense of this is that Cain said before G-d: “Behold, my sin is great, and You have punished me exceedingly, but guard me that I should not be punished more than You have decreed upon me for by being a fugitive and wanderer and unable to build myself a house and fences at any place, the beasts will kill me for your shadow has departed from me.” Thus Cain confessed that man is impotent to save himself by his own strength but only by the watchfulness of the Supreme One upon him.
*Look to the end: Kayin's effective argument was "if you made man have the capacity for evil, then you have to also allow repentance to fix our sins." Why even make a human who can do evil if not allowing tshuvah? Per Liat, Rav Chakhamim Ryaboy "why make temptations like the tree, if G-d won't allow repentance when mistakes are made?"
בתמיה. וחסר ה"א התימה כי לא יתכן בניחותא ופי' מנשוא על עצמו פי' שחטאו חטא גדול שאין ראוי לסליחה דאם כן היה מגדיל החטא על עצמו ואיך יאמר הן גרשת אותי הרי הודה על חטאו וראוי לזה. [רא"ם]: קשה מאי שייכות יש לזה לטעינות העון נראה דה"ק התחתונים ודאי חוטאים כמ"ש (להלן ח כא) כי יצר לב אדם רע וגו' א"כ למה בראת אותם אלא ע"כ דיש תקון לחטא התחתונים על ידי תשובה א"כ לעוני אי אתה טוען. [דברי דוד]:
An incredulous question. This is a question, although it lacks the ה which would indicate this. It cannot be a statement, with Kayin saying that his sin is unforgivable and more than he can bear, for Kayin would then be magnifying his sin. How could he then [contest his punishment and] say, “Behold, today You have banished me...” if he already admitted that his sin is deserving of it? (Re’m) Question: What does Hashem’s bearing of the worlds have to do with His bearing of Kayin’s sin? It seems the answer is: Kayin is saying that earthly creatures surely sin, as it says (8:21), “For the inclination of man’s heart is evil from his youth.” Why did You, Hashem, create them? It must be that repentance can rectify their sins. If so, “You cannot manage to bear my sin?” (Divrei Dovid)