It's not an assumption. That command is reiterated in different wordings throughout scripture. After the flood God said, “
Be fruitful and multiply and fill the earth. The fear and dread of you will fall on every living creature on the earth, every bird of the air, every creature that crawls on the ground, and all the fish of the sea. They are delivered into your hand. Everything that lives and moves will be food for you; just as I gave you the green plants, I now give you all things...."
Consider the part about filling the earth in the light of a mortal human. Can the earth literally be filled in a single lifetime? No. How many generations would it take to fill the earth and subdue it for God, turning what was desolate into a living, thriving alternative? One "age"? Two? How long would those ages be? So we see the assumptions are on the doubting side of that command, not the affirmative side. God told Adam and Eve to be fruitful and multiply and several generations, after God had wiped the earth clear of all humans but eight, He uttered the exact same directive to Noah and his ilk

. Would that constitute a new age or a continuation of the same old age? Where do we find the premise of any mention of "
age" asserted in either passage? Nowhere. So let's not create nonsense and imagine it reason.
Generations later God told Abram, "
I will make you exceedingly fruitful, and I will make nations of you, and kings will come from you." Two generations later He told Jacob, "
God also said to him, “I am God Almighty; Be fruitful and multiply; A nation and a multitude of nations shall come from you, And kings shall come from you." 400 years later He told the wandering Hebrews, "
So I will turn toward you and make you fruitful and multiply you, and I will confirm My covenant with you." Jesus reframes the dominion mandate as the great commission before he ascended, "
Go, therefore, and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit, teaching them to follow all that I commanded you." Millennia after God first spoke those words to Adam and Eve, Paul was preaching to the Greeks and said, "
He made from one man every nation of mankind to live on all the face of the earth, having determined their appointed times and the boundaries of their habitation..."
That is a sampling of the many passages found throughout scripture that speak of multiplying, being fruitful, overcoming desolation, and ruling or asserting authority as agents of God.... all over the planet. No assumptions needed, wanted, or made.
Now you have plenty of reasons for knowing God's first command has never been repealed.
And this is very op-relevant because everyone assumes Paul is writing about Adam eating the forbidden kiwi when he says, "...
through one transgression there resulted condemnation to all men..." but the facts of history are that had Adam obey the dominion mandate he never would have disobeyed the prohibition against eating the fruit from the tree of the knowledge of good and evil. Adam and Eve were given authority over all the creatures in the garden and the serpent was a creature in the garden. Had Adam ruled over that creature as he had been directed to do, the fall might never have occurred. The first sin was not the eating of the forbidden fruit. The first sin was not ruling and subduing the serpent. The first act of disobedience was a failure of dominion.
The very next episode reported in scripture is another example of failed dominion.
Genesis 4:7
If you do well, will your face not be cheerful? And if you do not do well, sin is lurking at the door; and its desire is for you, but you must master it.
Master sin. Even in the sinful state God expected Cain to master sin. Sin is just another version of desolation. When God made the earth it was desolate. God planted a garden on it and told the humans he made to multiply be fruitful subdue the earth and rule over it. The same conditions existed just before Christ died. When Jesus returned to Jerusalem, he was confronted by the Jewish leaders multiple times and each episode illustrates the pervasive desolation that existed there. By the end of the day he has pronounced
the dwelling of the Pharisees desolate. Isaiah, Jeremiah, Ezekiel, Hosea, Micah, and Zephaniah (and those are the ones I can think of in this moment) all spoke of the desolation that was Israel. Israel was supposed to be an example to all the other nations. They refused to be the proverbial light on the hilltop, the shining example, all others were to emulate but they failed to multiply, subdue, and rule. The history of much of the Bible is simply a replay of Genesis 3:6-7 in which Israel is constantly giving in to idolatry and adultery, subjugation instead of authoritative rule over all the desolation existing in the world.