Book 8 makes a sharper claim than “fallen people need forgiveness.” It says even if a person reached a high standard of perfection, the lineage problem would still remain.
That is why the chapter insists on engrafting. The issue is not only behavior or even maturity. The issue is the root from which love, life, and identity are being drawn.
This is a major theological distinction. A branch can be polished, disciplined, and even made fruitful for a time, but if it remains joined to the wrong root, its deepest inheritance has not changed. The Messiah is therefore needed not merely as teacher or forgiver but as the source of a new stock.
The force of the claim is pastoral as well as doctrinal: some problems are too deep to be fixed by effort alone because the root itself needs replacement.