Is it not a negative play. A score from a scrum isn't guaranteed but a penalty would give at least two points. I've not made up my mind fully because some blame lies with the player who delivers the final pass. But a premeditated knock down, as most are, surely should be punished

It's good defence, a split second reaction and it would be a bit harsh to concede a penalty by trying to prevent the opposition from scoring.

No different to deliberately slapping a ball out of play the in goal area to me, sacrificing possession to save a try!

I don't get this.
Just because Union have a strange rule for a deliberate knock on, it doesn't mean that we should follow suit.
In league there is very little reason to knock the ball on deliberately.
Ok if you are a single defender with 2 attackers coming at you, it's better to knock the ball on rather than concede a try.
However, the resulting scrum is sufficient advantage and there is no need to increase the "penalty" for such an infringement.

Also, there is nothing better to see than a player intercepting and going the length of the field and a change to the rules may just
reduce these moments and nobody should want that to happen.

Wasn't it Leeds who wanted the deliberate knock on rule introduced as their style of play was so predicatable, all the defences knew how to defeat them ?

in that case my bad. And he's spot on.