But the question is, why do you assume all people who say that Christians won't be sinless are using that "as an excuse that grace covers willful sin"? That's not very fair at all.
I know the difference between a person sinning in weakness and fear and ignorance, and a person sinning because they don't want, or care about the grace of God. But you don't seem to know the difference at all and have broad stroked all sin in all Christians into the category of the sin unto death.
Those who sin are slaves to sin.
Joh 8:34 Jesus answered them, Verily, verily, I say unto you,
Whosoever committeth sin is the servant of sin.
Rom 6:16 Know ye not, that to whom ye yield yourselves servants to obey,
his servants ye are to whom ye obey; whether of sin unto death, or of obedience unto righteousness?
Those who sin are of the devil.
1Jn 3:8
He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
Those who abide in Christ do not sin.
1Jn 3:6
Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither known him.
Those born of God do not produce sin.
1Jn 3:9
Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.
Jesus came to save people from their sins.
Mat 1:21 And she shall bring forth a son, and thou shalt call his name JESUS: for he shall
save his people from their sins.
Jesus gave Himself to redeem us from all iniquity and to purify a people.
Tit 2:14 Who gave himself for us, that he might
redeem us from all iniquity, and purify unto himself a peculiar people, zealous of good works.
Jesus can make you truly free.
Joh 8:36
If the Son therefore shall make you free, ye shall be free indeed.
When we crucify our old man with Christ and are raised up by that same power that raised Jesus from the dead we are set free from sin. Thus we no longer serve sin because if we serve sin we are slaves to sin. The sin stops.
Rom 6:6 Knowing this, that
our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin.
Rom 6:7
For he that is dead is freed from sin.
Being set free from sin we do not let it reign.
Rom 6:11 Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord.
Rom 6:12
Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof.
Rom 6:13
Neither yield ye your members as instruments of unrighteousness unto sin: but yield yourselves unto God, as those that are alive from the dead, and your members as instruments of righteousness unto God.
This is because sin does not have dominion over us because we are under grace.
Rom 6:14 For
sin shall not have dominion over you: for ye are not under the law, but under grace.
Grace which teaches us.
Tit 2:11 For
the grace of God that bringeth salvation hath appeared to all men,
Tit 2:12
Teaching us that, denying ungodliness and worldly lusts, we should live soberly, righteously, and godly, in this present world;
Grace which quickens us.
Eph 2:5 Even when we were dead in sins,
hath quickened us together with Christ, (by grace ye are saved;)
We stand in the grace which we access through a genuine faith. We don't access grace to fall. We walk in victory.
Rom 5:2 By whom also we have
access by faith into this grace wherein we stand, and rejoice in hope of the glory of God.
God gives us grace for obedience, because it is being a doer of the word, a worker together with God, so that grace is effectual.
Rom 1:5 By whom
we have received grace and apostleship, for obedience to the faith among all nations, for his name:
2Co 6:1 We then, as
workers together with him, beseech you also that ye receive not the grace of God in vain.
We walk by this power keeping out bodies in subjection, not yielding to the passions of the flesh in disobedience to God, rather we walk in total victory.
Rom 8:11 But
if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead
shall also quicken your mortal bodies by his Spirit that dwelleth in you.
Rom 8:12 Therefore, brethren, we are debtors, not to the flesh, to live after the flesh.
Rom 8:13 For if ye live after the flesh, ye shall die: but
if ye through the Spirit do mortify the deeds of the body, ye shall live.
Rom 8:14 For as many as are led by the Spirit of God, they are the sons of God.
That is what the Bible plainly says. Your disagreement is with God. People can use human reasoning and the traditions of men all they want to try and find some way to rationalize continued sinning while remaining saved but they are simply deceived. Satan teaches "Ye shall not surely die" and he appears as a lamb, a wolf in the clothing of a sheep, speaking the language of the dragon.
If someone is sinning in fear, weakness and ignorance then they need to yield to the light they have and go through a time of godly sorrow which will work a genuine repentance. Where they can lay aside all their filthiness and overflow of weakness, where they can be broken on the stone of Christ, where the fallow of the ground can be broken up. Then and only then will God raise that person up to newness of life and quicken their spirit back to life.