No one has ascended to heaven but He who came down from heaven, that is, the Son of Man who is in heaven.
There is no future reference in this scripture.
Both Paul and John were caught up to heaven after Jesus was Resurrected.
The book of Revelation shows others in heaven awaiting to return to earth with Jesus at the Second Coming.
To be absent from the body if to be present with thd Lord.
JLB
The future reference was when Jesus used Moses in vs. 14 as an example to explain to Nicodemus that he too had to ascend to the Father in order for all who will believe in him will have eternal life.
Nicodemus, like the disciples, knew Jesus as a teacher/prophet, but did not understand his fill mission. No one fully understood the earthly things Jesus was teaching and could never understand the Spiritual things of Jesus. Jesus was trying to explain to Nicodemus that as Moses lifted up the serpent in the wilderness that he also had to be lifted up (ascend) to the Father that everyone who believes in him will have eternal life with the Father. It would not be until Jesus ascended to the Father and
then came back to earth for forty days teaching giving that of the Holy Spirit that now their Spiritual eyes and ears were opened to fully understand that of what Jesus taught them.
John 3:9 Nicodemus answered and said unto him, How can these things be?the disciples
John 3:10 Jesus answered and said unto him, Art thou a master of Israel, and knowest not these things?
John 3:11 Verily, verily, I say unto thee, We speak that we do know, and testify that we have seen; and ye receive not our witness.
John 3:12 If I have told you earthly things, and ye believe not, how shall ye believe, if I tell you of heavenly things?
John 3:13 And no man hath ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven.
John 3:14 And as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up:
John 3:15 That whosoever believeth in him should not perish, but have eternal life.
These two verses are only saying it would be better to be present with the Lord than to be in home in the body. Paul, like all of us would rather be with Jesus right now, but have to remain in this fleshly body until Jesus returns.
2Co_5:6 Therefore we are always confident, knowing that, whilst we are at
home in the body, we are absent from the Lord:
2Co_5:8 We are confident, I say, and
willing rather to be absent from the body, and to be present with the Lord.
I have to go, but will return and explain that of Revelation.