Note: God didn’t instruct man on eating meat until he gave it to Noah after the flood.
I’m not vegetarian, but man may have been up to that point.
Also note that God specifically states that after the flood he’s only going to give man 120 years, although it was not an immediate limitation.
The announcement that "his days will be 120 years" means the clock was ticking for Noah to build God's Ark. The rain would start 120 years from that day.
IMHO, the reason mans years started to decline was the changes in the Earth after the Flood. The Earth could have been as much as 10% smaller due to the waters of the deep coming to the surface. Sorta like squeezing a sponge. The water in the atmosphere fell to earth leaving man exposed to UV rays, and on and on, with life shortening changes from the effects of the Flood. Some reckon the atmospheric pressure was cut in half and gravity changed.
The thing to take forward for me would be the "120 years" reference was not the estimated years of longevity, but a cut off point for Noah's Ark to be finished. The very name "Methuselah" meant in Hebrew when he died the end would come. Methuselah died and the rain started, all on God's timetable. Noah was 600 yo when the rain started. Could this be transposed into 6000 years of man and then God's judgement comes,...I don't know, but Noah and the Ark is a "type and shadow" of the rapture and the Judgement of God coming in the future. Many people haven't noticed that God told Noah to enter the Ark 7 days before the rain started. This is for the people that say we can't know the day or the hour, but Jesus will call for His Bride on some Rosh Hashanah in the future. That is the "Day no one knows" for a Jew as it lasts for 48 hrs. The church will be warned before God's judgement falls if you have "ears to hear". Luke 17:26-30. Not one drop of rain fell on Noah nor one rock of Brimstone fell on Lot. God's people will be brought into the safety of God's provision before His judgement falls on the Earth. 1 Thess 5:1 tells us the Jews know the day and hour the thief comes. Gentiles won't know because they don't reverence the Lord's Days spoken of in Leviticus 23.