The reason for the rule is to deter players slapping the ball down when they can't realistically stop try scoring chances in any other legitimate way. It is an absolutely essential law to preserve positive, skills-based attacking rugby which flows.
"Oh no I knocked it on did I? What a shame.. How will we ever recover from giving away a free kick..."
Is it? The other code manages fine without it. That's one of the reasons I see it as being overly punitive in our code.
Having followed both codes for a long time, IMO there isn't a disproportionately high number of deliberate knock downs in league. Of course there's more than we see in union simply because it's allowed, but not so many that it's spoilt the game.
In fact because defenders have more options available to them, it means attacking teams have to hone structures and individual skillsets to create greater space, and over the years that's driven innovation. Matt Gidley's back hand draw-pass and Andrew Johns' kick-pass, which they each pioneered in the mid/late 90s, are now commonplace.
You say the law
preserves positive, skills-based attacking rugby and ok, I see that perspective. Although if that's its sole intention you could argue its effectiveness..... but IMO axing it will
grow positive skill-based attacking rugby. And just as importantly, improve flow by removing a TMO intervention point.
If I could, I would kill it off tomorrow. And I absolutely believe we'd be immediately better off.
