There are silly "imaginations" that are nothing but mere fantasy or entertainment. We see this in the story telling of children at play, authors, and play writes.
Then there are "imaginations" that are speculative arguments that oppose the truth of the Gospel, such as what we see coming from philosophical atheism, paganism, etc.. These are to be destroyed:
2 Corinthians 10:5 New American Standard Bible (NASB)
5 We are destroying speculations and every lofty thing raised up against the knowledge of God, and we are taking every thought captive to the obedience of Christ,
Then there are "imaginations" that are foundational to understanding implications and the formation of new ideas or concepts not present to the senses: this is often called vision, creativity, innovation, resourcefulness.
There is no need to fear the word "imagination". But rather, there is a need to properly define what is meant by it.
So, back to my main point. If Adam and Eve had never fallen, and death and disease were not present, this "multiplication" factor would have exceeded the planet's ability to sustain mankind long ago. Clearly the statement in Genesis was meant to express God's desire to see humanity flourish and fill the earth. It wasn't a universal didactic command for every capable human family have at least four children.