I agree that according to John 3:13 the way to resurrection to life in heaven only became available till after the resurrection (alleged) of Jesus. The textual context of the verse is of before Jesus died and went to heaven (allegedly went to heaven), thus the text gives the impression that Jesus said the Son of Man ascended to heaven before Jesus was born (namely, allegedly before the Son of Man descended to be born as Jesus as a human). [Perhaps that verse is an anachronistic comment by the writer of the gospel, and thus that the writer should have used different wording to fit the temporal context of the rest of his gospel account.] But for many years as a JW and as an inactive JW (and ex-JW) the part, which seemingly is about the Son of Man having ascended before he descended, was very puzzling to me. That is because me, when I had thought of the ascension of Jesus to heaven, I thought that it (according to the Bible) only happened after Jesus became resurrected (according to the Bible). I did not notice any verses in the Bible which spoke of Jesus (or a divine Son of Man) being on Earth prior to Jesus being conceived in the womb of Mary (according to the Bible).
Granted the WT teaches that some of the OT statements about Jehovah coming down to Earth were of Jesus coming to Earth as the representative of Jehovah. Likewise the WT teaches that some of the OT statements about the angel of Jehovah coming down to
Earth were of Jesus coming to Earth as the representative of Jehovah. But for many years I never interpreted the verse in John 3:13 being about that, since the OT does not state that Jesus descended (such as having descended in BCE times) and it does not state that the Son of Man descended. But I now think that some 1st century Christians (and maybe some Jews living before the 1st century CE, ones who contemplated the arrival of the Messiah) might have interpreted parts of the OT mentions of YHWH coming down to Earth as being about a divine being who was not Yahweh in the sense of God the Father. I noticed that in some of the Psalms it is hard to identify which being is said to be speaking in a given verse; it seems like the identity of the 'speaker' changes from one verse to another in the same Psalm, without the change being explicitly stated/named in the Psalm. That makes such Psalms confusing to me. [I later learned that sometimes one of the 'speakers' might be a human chorus, or a human king sitting on 'David's throne'.] Some of those Psalms seem to be partly about the Messiah and partly about YHWH God. Maybe that is how the idea got started. I can also see how readers could have interpreted those verses as indicating that there are two divine persons called YHWH, and thus get a binitarian idea or a trinitarian idea.