What are you basing that claim on?
As with any skill, practice is necessary to achieve competence. If homework is no longer a viable way to force students to practice, you'll need to find some replacement, you can't just abolish it and expect there to be no negative consequences.
By what means? Have you got a magic wand that will turn 3rd graders into mature functional adults who will study on their own without homework?
> The study followed 27,000 pupils aged 12 to 18
(Third grade is 9-10 years old I believe.)