The Bible says, normal life and commerce at the pretrib rapture of Noah gathered pretrib.
In your model of post flood gathering, there is no normal life OR COMMERCE.
You completely ignore what Jesus indicated will happen to those who are left. To think that those who are left will return to normal life is beyond ludicrous. Just look at what Jesus actually taught will happen to those who are left behind.
He starts out by comparing what will happen on the day of His coming to what happened on the day Noah entered the ark and the day Lot went out of Sodom.
Luke 17:26 And as it was in the days of Noe, so shall it be also in the days of the Son of man. 27 They did eat, they drank, they married wives, they were given in marriage,
until the day that Noah entered into the ark, and the flood came, and destroyed them all. 28 Likewise also as it was in the days of Lot; they did eat, they drank, they bought, they sold, they planted, they builded; 29 But
the same day that Lot went out of Sodom it rained fire and brimstone from heaven, and destroyed them all. 30 Even thus shall it be in the day when the Son of man is revealed.
He specifically pointed out how the flood "destroyed them all" and how the fire that came down on Sodom "destroyed them all" and then said "Even thus shall it be in the day when the Son of man is revealed.". That means He was saying that He will destroy them all (all unbelievers) when He comes just as the flood "destroyed them all" and the fire on Sodom "destroyed them all".
Then He tells people to remember what happened to Lot's wife as an example of what will happen to those who are too attached to their lives in this world on the day He returns.
Luke 17:31 In that day, he which shall be upon the housetop, and his stuff in the house, let him not come down to take it away: and he that is in the field, let him likewise not return back. 32
Remember Lot's wife. 33 Whosoever shall seek to save his life shall lose it; and whosoever shall lose his life shall preserve it.
Anyone who cares more about their temporary lives on this earth than about being willing to sacrifice their lives for God will be killed when Jesus comes.
Then Jesus said this:
Luke 17:34 I tell you, in that night there shall be two men in one bed; the one shall be taken, and
the other shall be left. 35 Two women shall be grinding together; the one shall be taken, and
the other left. 36 Two men shall be in the field; the one shall be taken, and
the other left. 37 And they answered and said unto him, Where, Lord? And he said unto them,
Wheresoever the body is, thither will the eagles be gathered together.
So, keeping in mind what He had just previously said it shouldn't be hard to see that the ones taken will be taken up to meet the Lord in the air and the ones left behind will be killed just as Noah and Lot and their families were brought to safety in both of those scenarios while everyone else was killed in both scenarios.