This is apparent because the saints are there before the throne of God, but so is Jesus, so Jesus did not outright leave Heaven yet.
This is because you are still hung up on the timing of Revelation 7:9-14.
John is not "just seeing" as if they just arrived. John saw the final group, because at the 6th Seal, Second Coming the rest of the church was brought home to Paradise.
"For the Lamb which is in the midst of the throne shall feed them, and shall lead them unto living fountains of waters: and God shall wipe away all tears from their eyes."
The Lamb ascended with all the OT redeemed on Sunday morning after talking to Mary. That is when this verse became a reality. Great tribulation has been the last 1993 years since that morning. Steven was welcomed by the Lamb standing to greet Steven as he was being stoned.
What those people were doing in Paradise serving God day and night in His Temple has been true since 30AD.
The Lamb does not have to be in this scene at the time John is seeing this multitude. The Lamb was there from the Resurrection ascension until the 6th Seal was opened. All those in Christ are there even as we post these posts. The only ones not there are those who are still alive (in Adam's dead flesh) and remain on the earth. The reality of the church on earth stops at the Second Coming, the 6th Seal. The church will not return for over 1,000 years. That will happen in the NHNE when the New Jerusalem descends from Heaven to Earth. Paradise will be the New Jerusalem in the NHNE. The New Jerusalem is not even a reality right now. Paradise is the current reality.
What John sees as Heaven and earth fleeing away is the same as Paul's point in 1 Corinthians 15:24-28
"Then cometh the end, when he shall have
delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power..... For he hath put all things under his feet. But when he saith all things are put under him, it is manifest that he is excepted, which did put all things under him. And when all things shall be subdued unto him, then shall the Son also himself be subject unto him that put all things under him,
that God may be all in all.
Creation will return to God like it started out as nothing. Then the NHNE will be a totally different reality.
At Armageddon Jesus returns, only if Jesus left at the mid point of the week of the 7th Trumpet. From the 1st Trumpet until the 7th Thunder, is when His vesture is dipped in blood.
"And he was clothed with a vesture dipped in blood: and his name is called The Word of God."
All of Adam's flesh must be destroyed. Armageddon is just that last remnant of Adam's dead corruptible flesh.
During the Trumpets we see Matthew 25 and the separation of the sheep and goats. During the Thunders we see Matthew 13 and the tares and the wheat. In 30AD mankind symbolically shed the blood of the Lamb. From the 6th Seal to the 7th Trumpet the Lamb will be symbolically shedding the blood of Adam's dead corruptible flesh. That is why they are hiding in the caves and under the rubble. The wrath of the Lamb is come.
"And said to the mountains and rocks, Fall on us, and hide us from the face of him that sitteth on the throne, and from the wrath of the Lamb: For the great day of his wrath is come; and who shall be able to stand?"
No one can know how long the Trumpets and Thunders will last. Because no one knows when the 6th Seal will be opened. The 42 months between the 7th Trumpet and Armageddon is about all we know (at least that is what every one posts about). And we will not even know if those 42 months happen, until the days of the 7th Trumpet.