If we say that we have no sin, we deceive ourselves and the truth is not in us.
Notice, it does not say, "If we say that we do no sin, we deceive ourselves, and the truth is not in us.
If it said that, it would be contradicting immediate context (1 John 3:5-9).
1 John 1:8 is speaking of the element of indwelling sin.
We all have sin dwelling in us; however that element of sin can be rendered dead (Romans 6:6, Galatians 5:24, Romans 7:8) so that it no longer has any say over our behaviour (Romans 6:14).
We are not obligated to obey the flesh (Romans 8:12 (kjv, NLT)).
Therefore, when it comes to practicing or committing sin, it is possible to live a life of perfect holiness for an extended period of time; even for the rest of our lives (Luke 1:74-75); walking not after the flesh but after the Spirit (Romans 8:4) in a consistent manner for an extended period of time.
The doctrine of the inevitability of sin, for the believer, is therefore a false doctrine and a heresy.