The Fifth Seal (6:9-10) 9 When He opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the testimony which they held. 10 And they cried with a loud voice, saying, “How long, O Lord, holy and true, until You judge and avenge our blood on those who dwell on the earth?”
Notice that the martyrs are under the altar and it refers to their blood. This SIGN-POINTS TO the same OT imagery taken from the sacrificial system. In the earthly temple when the lambs and goats were slain by the priests their blood would stream down the altar and form into a pool around its base. This vision is symbolizing that the martyrs are none other than those saints who are being "sacrificed" on the altar of religion/Law because of the priesthood and corrupt temple system ORIGINATING IN JERUSALEM.
This makes no sense for the future because that imagery doesn't apply anymore. There is no temple anymore in Jerusalem in which the priesthood is instigating the murder of the saints.
Remember the persecution against the church in John's day is primarily coming from Jews and it is THEY who are receiving the wrath of God during that great tribulation:
1 Thessalonians 2:14-16, "14 For you, brethren, became imitators of the churches of God which are in Judea in Christ Jesus. For you also suffered the same things from your own countrymen, just as they did from the JUDEANS, 15 who KILLED BOTH THE LORD JESUS AND THEIR OWN PROPHETS, AND HAVE PERSECUTED US; and they do not please God and are contrary to all men, 16 forbidding us to speak to the Gentiles that they may be saved, so as always to FILL UP the measure of their sins; but WRATH has come upon THEM to the uttermost."
Revelation 18:24, "In her was found the blood of the prophets and saints, and of all who were slain on the earth."
He said 31 “Therefore you are witnesses against yourselves that you are sons of those who murdered the prophets. 32 FILL UP, THEN, THE MEASURE of your fathers’ guilt. 33 Serpents, brood of vipers! How can you escape the condemnation of Gehenna?34 Therefore, indeed, I send you prophets, wise men, and scribes: some of them YOU WILL KILL and crucify, and some of them you will scourge in your synagogues and persecute from city to city, 35 that on YOU may come ALL THE RIGHTEOUS BLOOD SHED ON THE EARTH, from the blood of righteous Abel to the blood of Zechariah, son of Berechiah, whom you murdered between the temple and the altar. 36 Assuredly, I say to you, all these things will come upon THIS GENERATION. (Mt 23)
Remember when Stephen was stoned, THE FIRST MARTYR? He saw Jesus STANDING (when Hebrews 1 says he is seated). Isaiah 3:13 says in the context of Israel's LAST DAYS... "The Lord will STAND to judge HIS people."
Commentator David Chilton says, “Like Cain, the “older brothers” of the Old Covenant envied and murdered their righteous “younger brothers” of the New Covenant. (cf 1 John 3:11-12)
Revelation is not about your future, it was about THEIR future. God has plans to advance his kingdom in the earth... he has a future and hope for the world.