Shibud mitzrayim: a necessary evil?

R. O. Seforno writes (Bereishis 46:3) that had the Israelites stayed in Canaan they would have intermarried and assimilated, never developing a distinct national or religious identity, but being shepherds—which was abominable to the Egyptians—they'd be given their own place to live and allowed to become a distinct national unit. 

אנכי האל אלהי אביך – אני הוא שאמרתי  "אל תרד מצרימה" (בראשית כ"ו:ב'), אני הוא שאומר אליך אל תירא מרדה מצרימה – אתה, וזה כי לגוי גדול אשימך שם – כי אמנם אם היו בניך יושבים פה היו מתחתנים בגויי הארץ ומתערבים עמהם, אבל במצרים לא יקרה זה, "כי לא יוכלון המצרים לאכל את העברים לחם" (בראשית מ"ג:ל"ב), ובכן יהיו לגוי נבדל, כאמרם זכרונם לברכה: "ויהי שם לגוי" (דברים כ"ו:ה'), מלמד שהיו מצוינים שם (ספרי שם). 

Ramban says that Hebrew was the spoken language in Canaan:

רמב"ן בראשית מ"ה:י"ב

ויתכן שאמר להם כך לאמתלא ולפיוס, כי איננה ראיה שידבר אדם אחד במצרים בלשון הקדש, כי על דעתי הוא שפת כנען. [כי אברהם לא הביאו מאור כשדים ומחרן כי ארמית היא, והגל הזה (בראשית ל"א:מ"ז-מ"ח) עד, ואיננו לשון לאיש אחד לבדו, אבל הוא שפת ארץ כנען] ורבים במצרים יודעים אותו, כי קרוב הוא, ואף כי המושל, שדרך המלכים והמושלים לדעת הלשונות. 

Although spoken by many and definitely by the aristocracy it was not the lingua franca of Egypt which was Egyptian and would further help to keep them apart as they developed their unique national-religious identity 

Rambam writes (Moreh 3:32) that people are slow to change and that God and the Torah are geared to work with human nature as it is. 

No reason is provided for why Avraham's progeny had to be subjugated in a foreign land for so long—it is delivered as a matter of fact: 

בראשית ט"ו:י"ג

וַיֹּאמֶר לְאַבְרָם יָדֹעַ תֵּדַע כִּי גֵר יִהְיֶה זַרְעֲךָ בְּאֶרֶץ לֹא לָהֶם וַעֲבָדוּם וְעִנּוּ אֹתָם אַרְבַּע מֵאוֹת שָׁנָה. 

There's nothing quite as unifying and effective at creating a tough and durable new national religious identity that is opposed to reigning popular culture than having a group face adversity over a long stretch of time, particularly on foreign soil, enabling the people to grow into a distinct nation, with its own language and raison d'etre, pushing it to develop and gel into a new and distinct nation, with its own unique character and mission statement. 

Perhaps no specific sin or crime is necessary to have caused shibud mitzrayim (altho many are posited) but rather that starting a radical new movement and nation of ethical monotheism by definition requires certain conditions to allow it to launch and escape the gravitational pull of the surrounding customs and beliefs. 

The new religion that Avraham was chosen to found perhaps required the conditions provided by shibud mitzrayim to purge it from residual canaanite dross (Devarim 4:20) and to enable the fledgling and nascent monotheistic family-nation to develop a strong and durable identity that would equip it to withstand millenia of alien and pagan winds and cultures, not to mention expulsions, massacres and persecutions, and still stay true to its national calling. 

R. Yaakov Emden writes (here and ff) that  Israel's survival throughout the ages is a greater miracle than all the open miracles in our history. 

Not to detract from its miraculousness however I would add that for a people to survive like that with it's national religious indentity intact also requires an unbreakable national consciousness which years of consistent persecution in a foreign country would go a long way toward forging.

B BB 15a-b records many opinions as to when Job lived, if at all. 

One view—a tannaitic one—is that his life spanned from when the Israelites went into Egypt until they emerged:  

יְמֵי שְׁנוֹתָיו שֶׁל אִיּוֹב מִשָּׁעָה שֶׁנִּכְנְסוּ יִשְׂרָאֵל לְמִצְרַיִם וְעַד שֶׁיָּצְאוּ

Perhaps the significance is that the Israelites felt themselves unjustly persecuted, as did Job, and started to entertain his very questions regarding God's justice. 

Only after having gone through the shibud and exiting as a newly forged monotheistic nation did they begin to understand that it was never about their sins but rather about who and what the world needed them to become. 

Like Job, who God Himself proclaimed blameless, Israel came to understand that sometimes suffering functions teleologically rather than judicially: its purpose is human expansion and maturation, not divine retribution.