Let's look at 1 John closer, shall we?
1 John 5:13 tell us the letter was written to believers to assure their salvation.
John 3:14 and John 5:24 tell us that judgement is of no affect to the believer and that death is behind us! This should set off bells that we can't do something to put death in front of us again.
Why did John write this letter? There is something at work in the church that is threatening their salvation, hence his reason (1 John 5:13) for writing it. In 1 John 4:1-3 we are given an example of what these false teachers are teaching. What did they do? They were separating Jesus from his flesh, hence John's "Jesus Christ has come in the flesh" statement.
1 John 3:7 is another warning. These teachers were teaching the Christian life but not living it. So we have them called out by John on both the flesh of Christ as well as the Spirituality of Christ! They were trying to argue the Spiritual without the flesh (IE, living out the righteousness and also separating the flesh from Christ).
Now... That's the big problem John is addressing. How does he address it?
First of all, in 1 John 4:2, he says that Christ has come in the flesh. Not: came. But: come. He IS in the flesh, not separated. Christ is incarnate forever.
As already pointed out in 1 John 3:7, as well as in 1 John 2:29, he says the spiritual life must be accompanied by the spiritual actions.
1 John 3:9, and the verses that you are arguing from, John addresses more proves, anyone who PRACTICES sin is not of Christ.
What else does John do? He rejects the lie and false teaching that being "born again" equates to "sinlessness". Remember, the false teachers were separating the spiritual and the flesh, probably teaching sin was perfectly OK because it was just the flesh, and not the spiritual. (1 John 3:7) Their claims were that Christians did not ever sin. John addresses this in a few places.
1 John 1:8. WE... The BELIEVERS. If we say we ARE, we exist, we presently, have no sin, we are liars.
1 John 2:1. WE... The BELIEVERS. If we do sin, we have an advocate! John does not claim if you sin, you are not born again. Nay, he says an advocate exists! And only those (WE) who are Christians have an advocate. This is key.
1 John 5:16-17. There is a sin that does not lead to death. This is why you can see your BROTHER (believer) committing this sin, sin a believer commits does not lead to death. In John's example, this person is sinning. This person is not headed to death. And yet, HE IS YOUR BROTHER, a BELIEVER!
So how is the believer instructed to deal with this if they do?
1 John 1:9. Confession and cleansing.
1 John 2:1. We run to our advocate and return back to Him.