"Then" is a time separator. The dead in Christ would rise first - vs. 16. The context is of their concern about those who had already died before Christ returned (vs. 13-14).
"for this to you we say in the word of the Lord, that we who are living -- who do remain over to the presence of the Lord -- may not precede those asleep," (1 Thess. 4:15, YLT)
The Christians at Thessalonica expected Christ's return in their lifetime, or they would not have been asking Paul how their family and friends who had already died would be taken care of. His return in that century was the background of this question. Paul was assuring them that those who had already died in the Lord would be first to be risen from Hades - the realm of the dead (Luke 16:22-31).
Those who remained over to the presence (coming) of the Lord would not go before those who "slept". The "then" separator is life. The living must still face death first. It has always been that man must die and then he will face judgment. 1 Thess. 4 is part of the process of Heb. 9:27.
"and as it is laid up to men once to die, and after this -- judgment," (YLT)
Not as the KJV has it "the judgment", but just judgment. God has always judged each individual at their death, otherwise there would not have been that separation of Paradise and Tartarus in the realm of the dead, that prison called Hades.
"and Abraham expireth, and dieth in a good old age, aged and satisfied, and is gathered unto his people." (Gen. 25:8, YLT)
"And these [are] the years of the life of Ishmael, a hundred and thirty and seven years; and he expireth, and dieth, and is gathered unto his people;" (Gen. 25:17, YLT)
"And Jacob finisheth commanding his sons, and gathereth up his feet unto the bed, and expireth, and is gathered unto his people." (Gen. 49:33, YLT)
As also was Aaron (Num. 20:24), and Moses (Deu. 32:50), and David (1 Kings 2:10), and Josiah (2 Kings 22:20) and many others.
The realm of the dead was the prison with gates that could not hold Christ (Acts 2:27, 31), and into which Christ went after His death (Luke 23:43) with the thief on the cross. He preached to those in that prison (1 Pet. 3:19) in paradise. Christ held the keys to the gates of Hades (Rev. 1:18). And He gathered all of those dead out of Hades at His coming in the first century AD at the destruction of Jerusalem (Matt. 25:31-46) which was the follow-on discussion from Matt. ch. 24, and part of the "all things" that were fulfilled in that generation at the destruction of the temple.
He took those that had already died out of Hades at that time (AD 70), separating the sheep in paradise (Abraham's Bosom) and the goats out of Tartarus.
"Then" those that remain at His coming... those that had not yet died.. live until they die, and at their death, if they die in the Lord (Rev. 14:13) "then" they will be taken up into heaven to be with all the rest of the saved.
Life goes on. Life continued on after His return in that generation. We must all live our lives on this earth realm, and those that die in the Lord follow the process of being subject to the judgment at our death that has always occurred. The only difference is that now, since AD 70 that prison of Hades is gone. Christ threw it into the lake of fire after the destruction of Jerusalem, and after the separation of the dead out of Hades.
Now, those that die in the Lord are resurrected, changed in a twinkling of an eye (1 Cor. 15:52), and ascend at our death to be with all the rest in the Lord in heaven. Those souls that die without being covered by the blood of Christ face condemnation for having refused the most precious gift from God, and will be cast out (Matt. 22:13).
1 Thess. 4:17 was not speaking of a simultaneous "rapture" with those that had already died. Christ's resurrection changed the process. Those that were Christ's at His coming in AD 70 were those that had previously died and were waiting in paradise in Hades. Then those that remained, those who had not yet died would be taken up one by one at their deaths. The only corporate "rapture" was for those souls who were waiting in that realm of Hades, which no longer exists.
Please see "The Gathering of The Elect" at my blog here, also "The Resurrection In Three Parts" here.