Again, I think we are confusing two things here. No one is arguing that people get completely saved. If people couldn't get "completely" saved, then no one would ever be saved.
The question is, does everyone "stay" saved?
A house can be finished, just to be burned up or demolished later. A car can be restored to car-show quality, just to be wrecked later. A person can have their heart changed, and be turned to God, but...
2Pet 2:20; For if, after
they have escaped the defilements of the world by the knowledge of the Lord and Savior Jesus Christ, they are again entangled in them and are overcome, the last state has become worse for them than the first.
2Pet 2:21; For it would be better for them not to have known the way of righteousness, than having known it,
to turn away from the holy commandment handed on to them.
2Pet 2:22; It has happened to them according to the true proverb, "A DOG RETURNS TO ITS OWN VOMIT," and, "A sow,
after washing, returns to wallowing in the mire."
Heb 6:4; For in the case of those who have
once been enlightened and have tasted of the heavenly gift and have
been made partakers of the Holy Spirit,
Heb 6:5; and have tasted the good word of God and the powers of the age to come,
Heb 6:6; and
then have fallen away, it is impossible to renew them
again to repentance, since they
again crucify to themselves the Son of God and put Him to open shame.
The question isn't, does God have to power to save them completely, but rather do they have the power to fall away afterwards.
1Tim 4:1; But the Spirit explicitly says that in later times
some will fall away from the faith, paying attention to deceitful spirits and doctrines of demons,
1Tim 1:19; keeping faith and a good conscience, which some have rejected and
suffered shipwreck in regard to their faith.