Then the LORD said, "My Spirit will not contend with man forever, for he is mortal; his days will be a hundred and twenty years."
God will only put up with man in mortal flesh for 120 years, not forever according to the translator.
The word for 'days' here is 'yom,' which can mean a day or a space of time. The word for 'years' is 'shaneh' which can mean a space of time or a revolution of time. The translator has chosen 'years' for the space of time here because the translator thinks God is talking about one man's lifetime.
What makes the most sense to me here is that these are not years but 120 jubilees, or 120 times 50, or 6000 years.
At 6000 years, the sons of God will be revealed. And, God will no longer have to strive with man's mortal flesh.