will not contend with: Many think that this is an announcement of God’s decision to withdraw the restraining influence of his Spirit from human society and allow human wickedness to fully develop. Others think it means God would take away his life-giving breath from humans at an earlier age. The Hebrew word ruakh can mean both "spirit" and "breath"(see 6:17; 7:22; see also Psalm 104:29–30).
his days shall be 120 years: This might be a new limit on how long people could live. But after the great flood, people lived longer than 120 years (see, for example, Genesis 11:10–26).
Or, this might refer to a 120-year period of grace before the great flood (see Jonah 3:4; Matthew 24:37–38; 1 Peter 3:20; 2 Peter 2:5).