See https://biblehub.com/commentaries/1_thessalonians/4-15.htm
Opinions are not unanimous, but the idea is widespread among commentators that Paul did not place himself with certainty but merely with hope among those who would be alive when Christ returns. So how should we judge Paul? As an optmimist - not a false prophet. Take Paul's hope together with this prophecy in Acts 21:
10 After we had been there a number of days, a prophet named Agabus
came down from Judea. 11 Coming over to us, he took Paul’s belt, tied
his own hands and feet with it and said, “The Holy Spirit says, ‘In
this way the Jewish leaders in Jerusalem will bind the owner of this
belt and will hand him over to the Gentiles.’”
Combine the blessed hope of meeting Christ while alive with the prophecy that continued faithful service will lead to imprisonment and death, and you get a picture of a man of courage and faith, who like Job refused to let go of the Word of God even if God slays him, and was content to be struck down rather than abandon his savior.
Benson Commentary:
It is well observed, says Whitby, by the Greek scholiasts, that the
apostle speaks these words, not of himself, but of the Christians that
should be found alive at the second coming of Christ: so Chrysostom,
Theodoret, Œcumenius, and Theophylact; for he well knew that he was
not to live till the resurrection
Barnes Notes:
From this expression, it would seem, that some of the Thessalonians
supposed that Paul meant to teach that he himself, and many of the
living, would survive until the coming of the Lord Jesus, and, of
course, that that event was near at hand. That this was not his
meaning, however, he is at special pains to show in 2 Thessalonians
2:1-10.
Gill's Exposition of the Entire Bible:
That we which are alive, and remain unto the coming of the Lord: not
that the apostle thought that he and the saints then in the flesh
should live and continue till the second coming of Christ; for he did
not imagine that the coming of Christ was so near, as is manifest from
2 Thessalonians 2:1 though the Thessalonians might take him in this
sense, which he there corrects; but he speaks of himself and others in
the first person plural, by way of instance and example, for
illustration sake; that supposing he and others should be then in
being, the following would be the case: and moreover, he might use
such a way of speaking with great propriety of other saints, and even
of those unborn, and that will be on the spot when Christ shall come a
second time; since all the saints make up one body, one family, one
church and general assembly; so that the apostle might truly and
justly say, "we which are alive"; that is, as many of our body, of our
family, of our church or society, that shall be living at the coming
of Christ; and he might choose the rather to speak in this form,
person, and tense, to awaken the care, circumspection, diligence, and
watchfulness of the saints, since it could not be known how soon the
Lord would come: however, from hence it appears, that there will be
saints alive at Christ's second coming; he will have a seed to serve
him till he comes again; he always had in the worst of times, and will
have, and that even in the last days, in the days of the son of man,
which are said to be like those of Noah and of Lot: and these are said
to "remain", or to be "left", these will be a remnant, the residue and
remainder of the election of grace, and will be such as have escaped
the fury of antichrist and his followers, or of the persecutors of the
saints
Geneva Study Bible:
He speaks of these things, as though he should be one of those whom
the Lord will find alive at his coming, because the time of his coming
is uncertain: and therefore every one of us ought to be in such a
readiness, as if the Lord were coming at any moment.
Meyer's NT Commentary:
Meyer Lists the names of commentators who argued that Paul referred to the latter church as a collective, which may or may not include him as a survivor to the time of Christ's return:
Chrysostom, Theodoret, John Damascenus, Oecumenius, Theophylact,
Erasmus, Castalio, Calvin, Musculus, Bullinger, Zanchius, Hunnius,
Balduin, Vorstius, Cornelius a Lapide, Jac. Laurentius, Calixt, Calov,
Joach. Lange, Whitby, Benson, Bengel, Flatt...
Cambridge Bible for Schools and Colleges:
that we which are alive and remain unto the coming of the Lord - This should be: we that are alive, that remain (or survive) unto the
coming of the Lord. The second designation qualifies the
first,—“those (I mean) who survive till the Lord comes.” St Paul did
not count on any very near approach of the second Advent: comp. 2
Thessalonians 2:1-2. At the same time, his language implies the
possibility of the great event taking place within his lifetime, or
that of the present generation. This remained an open question, or
rather a matter on which questioning was forbidden
Pulpit Commentary:
That we which are alive and remain unto the coming of the Lord. These
words are the occasion of an important discussion. It has been
affirmed that the apostle here asserts that he himself expected to be
alive, with the majority of those to whom he was writing, at the
Lord's advent; that, according to his expectation, Christ's second
coming was close at hand. "Those who are alive and remain" are
distinguished from "those who are asleep," and in the former class the
apostle includes himself and his readers. And a similar declaration is
contained in the First Epistle to the Corinthians: "We shall not all
sleep, but we shall all be changed" (1 Corinthians 15:51). Such is the
view adopted by Grotius, Olshausen, Koch, Neander, Lechler, Baur,
Winer, Reuse, Lunemann, Riggenbach; and, among English divines, by
Alford, Jowett, Stanley, and Conybeare. Some of them suppose that Paul
changed his opinion on this point - that whilst in his earlier
Epistles he taught the immediateness of the advent, in his later
Epistles he renounced this hope and looked forward to his own
departure. There does not seem to be any ground for this opinion. On
the contrary, it would appear from the Second Epistle to the
Thessalonians, written only a few weeks after this Epistle, that Paul
did not expect the advent immediately, but mentions a series of events
which would intervene before its occurrence (2 Thessalonians 2:1-3).
And in this Epistle he represses the curiosity of the Thessalonians
about the precise time of the advent by telling them that it was
beyond the sphere of his teaching (1 Thessalonians 5:1, 2). We
consider, then, that the apostle speaks here as a member of the
Christian body, and uses a very common form of expression - that we
Christians which are alive and remain unto the coming of the Lord; but
not at all intending to express his confidence that he himself and his
converts would be actually alive at the advent. "He spake," says St.
Chrysostom, "not of himself, but of Christians who would be alive at
the day of judgment." Such is the view adopted by Chrysostom, Calvin,
Bengel, Hofmann, Lunge, Macknight, Ellicott, Bishop Alexander,
Wordsworth, and Vaughan.
