Q. As a nation, did Israel sin against God by remaining in Egypt for 400 years and not returning to the promised land? I mean, 400 years is a long time! Maybe they got way too comfortable. I realize He spoke to Abraham about this and we all know about God’s deliverance etc., but perhaps God punished Israel with Egyptian slavery for her failure to return? Also, we know that Abraham sojourned in Egypt but he didn’t stay. What do you think?
As I read the biblical accounts of ancient Israel’s time in Egypt (found in the books of Genesis and Exodus), I see first that Joseph, who brought his father Jacob and his whole extended family down to Egypt, told his brothers when he was dying, “God will surely come to you and bring you up out of this land to the land that he swore to Abraham, to Isaac, and to Jacob.” In other words, the Israelites were supposed to wait for God to come and give them just as clear an indication that they were meant to leave as they had gotten to come in the first place. So it wasn’t a sin for them to stay.
I see next that at the start of the following generation, “A new king arose over Egypt, who did not know Joseph.” This king was so concerned that the Israelites, who were already becoming a large community, might side with their enemies that he persuaded his people and officials to enslave them. So the Israelites didn’t have an opportunity to return to the land of Canaan but were too complacent to take advantage of it. Instead, they never got such an opportunity, because Joseph said when he was dying that they should wait, and the next thing that happened was that they were enslaved and trapped.
We should also note that in Exodus, God never says that He has punished the Israelites with slavery because they have complacently ignored their responsibility to return to the promised land. Instead, God says that He will punish the Egyptians for enslaving them and exploiting their labor. You mentioned that God had spoken to Abraham about the future enslavement of his descendants; God specifically told him, “For four hundred years your descendants will be strangers in a country not their own and they will be enslaved and mistreated there. But I will punish the nation they serve as slaves, and afterward they will come out with great possessions” (that is, as compensation for their unpaid labor).
It’s still an important biblical warning for us not to be complacent but instead to remain aware of what God expects of us and to seek eagerly to fulfill God’s purposes for our lives. Paul wrote to the Ephesians, for example, “Be very careful, then, how you live—not as unwise but as wise, making the most of every opportunity, because the days are evil. Therefore do not be foolish, but understand what the Lord’s will is.” However, the time that the ancient Israelites spent in Egypt doesn’t seem to be a case study of the kind of complacency we need to be careful to avoid.
Good Question 
That’s excellent. Thank you very much!