Wonderment
While indeed this is the main reason, Trinitarians do not have to explain Jesus' kind of "subordination" exclusively from His humanity: it can exist even if we consider that He is the Son, and the Father is the Father. This does not imply a difference in divinity, but only voluntary (love-related) subordination resulting from the order of origin. Another explanation could be what ancient ecclesiastical authors called "economy of salvation," which they understood as mutual adaptation for the sake of salvation in action and operation. For example, when we read about the Son's incarnation that "He did not consider equality with God something to be used to His own advantage, but emptied Himself," etc., it was not some ontological inferiority that compelled Him, but considerations necessary for the salvation of mankind that moved Him.
It's in vain to deny it: The New Testament itself explicitly teaches Jesus' dual nature. On the one hand, Jesus is called the only-begotten God (John 1:18), and God who was with the Father in the beginning (John 1:1) - on the other hand, it confesses that He became flesh (John 1:14), similar to us, to destroy the devil through death. So what kind of talk is it when JWs claim that the New Testament does not know anything about a "dual nature"? Of course, Trinitarians do not say that "the God of Jesus' human nature is the Father" - but that the God of Jesus, the God-man, is the Father, in terms of his human nature. What they bring up after this is a typical case of "arguing from silence": "Jesus does not say, 'The Father is greater than I am in my human nature'" - and can be easily strengthened with what we find in Philippians 2:5-8:
"though he existed in the form of God, did not regard equality with God as something to be grasped, but emptied himself, taking the form of a slave, assuming human likeness. And being found in appearance as a human, he humbled himself and became obedient to the point of death — even death on a cross."
Here, the state of being of God is explicitly attributed to Jesus as an existing state, and the word God is without an article just as much as when mentioning equality with God in the next verse. Therefore, it is entirely natural to refer both mentions of "theos" to the same thing. In addition, equality with God appears as something instead of which Jesus became human, so we must imitate this self-sacrificing mindset. But Paul does not emphasize that we should not strive higher than we deserve, but that we should not even seek what is rightfully ours, and consider others superior to ourselves (Philippians 2:3). From this, it is highly probable to take the word "harpagmos" (booty) in the sense of "res rapta" (seized thing), and not what your translation suggests, namely that Jesus did not want to seize equality with God.
Every attempt to render "harpagmos" here with "seizing" or a similar action and argue on this basis that Jesus "did not entertain the idea of usurpation to become equal with God", or as the NWT renders it: "did not even consider the idea of trying to be equal to God." is entirely fruitless. The word "hegeomai" does not mean "to consider," but "to regard as." It has a well-defined complement in Greek, which is grammatically expressed with a double accusative. In light of this, the above interpretation would lead here: "Jesus did not regard being equal with God as robbery" - which would grammatically mean exactly the opposite of what JWs want to get out of it: that is, he considered it something that is due to him. This is the basis for Furuli's argument, who wants to exploit that the "-gmos" suffix primarily creates active-minded nouns.
What JWs confidently claim is also not true, namely that there is no biblical evidence that "this is to be understood only in terms of His human nature". For the Epistle to the Hebrews neatly summarizes how much His becoming lower can be attributed to His being human (5,7-9):
"In the days of his flesh, Jesus offered up prayers and supplications, with loud cries and tears, to the one who was able to save him from death, and he was heard because of his reverent submission. Although he was a Son, he learned obedience through what he suffered, and having been made perfect, he became the source of eternal salvation for all who obey him"
That is, if He had not become human, He would not have needed to pray to the Father, nor to learn obedience, as He was not forced to do so as the Son. Otherwise: If from the beginning the Son were just a creature, so ontologically inferior to the Father regardless of his incarnation, why did he only have to "learn" obedience "in the days of his [being] flesh"? And Col 2:9 clearly proves that Jesus possessed the fullness (pleroma) of the deity (theotes, and not theitotes), not just some kind of demigod, lesser god ("a god") "quality".