The police have always had the option not to press charges in cases of assault. By having a “reasonable force” clause, this particular class of assault had a higher bar before charges where brought, and the defense had a means of arguing for the case to be dropped.
It's now a much more level playing field. The police can now pursue more prosecutions in cases where, in the past, it would have come down to, they say reasonable force disciplining their children, we say parent with an anger management problem.
It also goes a long way to setting the public attitude to one of, ‘it's not ok to hit your children’. That counts for a lot.
That case you allude to is a pretty good example of the shift in public perception that's needed here: Jimmy Mason was convicted of assaulting his child by punching him in the face, and the judge made it clear that this charge would have stood even under the old law. Yet last year all the usual suspects where shocked and appalled that he was charged with anything in the first place.