You ignore what Paul said in chapter 7. It is both. You have to take all scripture on a subject matter in context to understand what the Bible says on a particular topic. You don't pick and choose sections and ignore the other sections you don't want to look at. While our mind wants to be centered in Christ our sin nature is there all the time. If it wasn't we could live a life without sin. Are you sin free? If not you have that sin nature Paul is talking about. That section was in the present tense so it was his continuous battle the same as all of us have.
My spirit is sin free, my mind and body is not. My spirit is perfect,my mind and my body is not.
To look at Romans 6-8 correctly you need to understand man is a 3 part being and that his spirit, not his soul or body is made perfect at conversion. Why do you suppose Paul said the old man died? in so many of his epistles?
The mind, will, emotions and passions of a mind rule him unless they are renewed by the word of God and are led by his spirit.
The body will obey the mind or the spirit of man.
Romans 7 shows us the struggle Paul had with sin under the Mosaic Law . When were you ever under the Mosaic law?
Romans 7 shows us that if we have been freed by the law of the spirit of life in Christ Jesus and let the Spirit rule us, we overcome things, even sin now, not later. The man of sin is freed under Jesus Christ. Also, what do you suppose Peter was talking about in 2 Peter 1:11 about being a partaker of the divine nature?
If you at war with 2 different natures all the time, Paul says you are not saved. He says light and darkness cannot have fellowship. The old man and new Man cannot share the same house. This is being double minded and unstable.
Either you are a new man, that will still miss the mark at times and belong to Christ and crucified the flesh and all its passions and walk in the Spirit
OR
You are an old man that does not belong to Christ and have not crucified the flesh with all it's passions and still walk in the flesh, think in the flesh and are hostile to God and cannot please God.
Romans 8:10 says the body is dead to sin, but the spirit alive to righteousness.
Once again, we are not sin free and will still sin, but the old man[sin nature] died with Christ and the new man [spirit] was raised into newness of life. We walk have control over sin because we renew our minds, pull down every thought against Christ and are led by the Spirit of God.
When you get saved you get your choice back, regarding sin and the old life and partaking of the divine nature and life more abundantly . You go from sinner to sin forgiven child of light, who will still sin, but not be entangled, slowed down or controlled by sin.
This is Paul's argument in Romans 6-8. If oyu do not read it this way you will miss the truths contained in it, that sets you free from a life of double minded sin-consciousness false teaching.