Sorry mate. NIV (or NLT) by its lonesome is not QED. No other translation says "sinful nature".
Let's examine the Koine Greek, shall we?
(Rom 7:18) οἶδα γὰρ ὅτι οὐκ οἰκεῖ ἐν ἐμοί, τοῦτ᾿ ἔστιν ἐν τῇ σαρκί μου, ἀγαθόν· τὸ γὰρ θέλειν παράκειταί μοι, τὸ δὲ κατεργάζεσθαι τὸ καλὸν οὐχ εὑρίσκω.
σαρκί translates to flesh which is the term overwhelmingly used by all translations except for NIV & NLT.
Stop being a Platonist. It is not nature that sins, it is us as a person who sins. God did not create a sinful nature. He is not a pagan god that creates sinful things. Our human nature came straight from God. God does not produce sinful anything, let alone a sin nature. Only pagan gods produce sinful things. It is us, as persons, who sin and degrade ourselves. Granted, our bodies do decay and die but that is a consequence of Adam's sin and not because God is in the business of creating sinful nature.
If human nature is sinful then what does that make Jesus who took up the very same human nature we all have?
Newsflash! It's not human nature that sins. People sin.
Yep.
It is found at least 160 times in the Greek OT is always a reference to "flesh". It is used interchangeably with animals and all creatures of the earth. It is the reason that Paul appeals to how all of creation is subjected together unwillingly in "hope" of redemption.
Rom 8:19 For the earnest expectation of the creature waiteth for the manifestation of the sons of God.
Rom 8:20 For the creature was made subject to vanity, not willingly, but by reason of him who hath subjected the same in hope,
Rom 8:21 Because the creature itself also shall be delivered from the bondage of corruption into the glorious liberty of the children of God.
Rom 8:22 For we know that the whole creation groaneth and travaileth in pain together until now.
Rom 8:23 And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body.
Adam declared that Eve was now "bone of my bone", "FLESH of my flesh". An appeal to the Incarnation of Christ. "God among us in flesh".
Col 2:11 In whom also ye are circumcised with the circumcision made without hands, in putting off the body of the sins of the flesh by the circumcision of Christ:
It is irrevocably linked to a "bodily" form. Notice the words "body of the sins of the flesh". Taken from the ashes of destruction. Adam was fabricated incomplete. "A work in progress". When God said "let us make man in our own image" is written in the very Character of God. A work still yet incomplete until our bodies/flesh is changed. Our perfection is not complete without the Resurrection of the dead in Christ.
Heb 11:40 God having provided some better thing for us, that they without us should not be made perfect.
This world have never been the home of humanity. We are a heavenly people. A purposed royal priesthood of believers. Seeds planted in a dying garden.
1Co 15:39 All flesh is not the same flesh: but there is one kind of flesh of men, another flesh of beasts, another of fishes, and another of birds.
1Co 15:40 There are also celestial bodies, and bodies terrestrial: but the glory of the celestial is one, and the glory of the terrestrial is another.
1Co 15:41 There is one glory of the sun, and another glory of the moon, and another glory of the stars: for one star differeth from another star in glory.
1Co 15:42 So also is the resurrection of the dead. It is sown in corruption; it is raised in incorruption:
1Co 15:43 It is sown in dishonour; it is raised in glory: it is sown in weakness; it is raised in power:
1Co 15:44 It is sown a natural body; it is raised a spiritual body. There is a natural body, and there is a spiritual body.
1Co 15:45 And so it is written, The first man Adam was made a living soul; the last Adam was made a quickening spirit.
1Co 15:46 Howbeit that was not first which is spiritual, but that which is natural; and afterward that which is spiritual.
1Co 15:47 The first man is of the earth, earthy: the second man is the Lord from heaven.
1Co 15:48 As is the earthy, such are they also that are earthy: and as is the heavenly, such are they also that are heavenly.
1Co 15:49 And as we have borne the image of the earthy, we shall also bear the image of the heavenly.