The New Covenant is a covenant of faith. Maybe that point isn’t stressed enough, but when I’ve looked at the Bible I see that God takes covenants quite seriously. Cozying up to God and entering a covenant with Him then abandoning the covenant isn’t something He just accepts and moves on quickly from.
Psalm 78 gives us a play-by-play of how God distributed consequences, the removal of promises, and blessings for disobedience in the Old Covenant. Is God the same now? I believe so, yes, but God does not quit trying to reclaim His lost sheep.
A covenant is an agreement between and individual and God. I know that often what attracts people to Christ is the salvation and blessings, but it’s so much more than that. God expects people to keep their faith in Him because it’s a covenant of faith with Christ as the mediator.
I would be very careful merely accepting that if someone who genuinely came to the faith then genuinely abandoned the faith would be saved.
Consider this bit in 2 Peter 3 where Peter was addressing his brothers, sisters, and friends urging them to come to either come to repentance or perish:
2 Peter 3:8,9
8But do not forget this one thing, dear friends: With the Lord a day is like a thousand years, and a thousand years are like a day.
9The Lord is not slow in keeping his promise, as some understand slowness. Instead he is patient with you, not wanting anyone to perish, but everyone to come to repentance.