I get the OP's point but protecting the halves is an eternal struggle for all sides. It's not just about having an enforcer, it's about having a synchronised defensive structure that moves together, gets off the line quickly and knows how to judge attacking structures. It's very difficult for any backrower or such to support the halfback if the fundamental weakenesses of the defence permit the halfback to be regularly isolated.
I think Souths are one of the better sides at this defensive work, because Reynolds and Walker are hardly defensive powerhouses, but Souths are very good at not being drawn out of shape. This was particularly evidenced in the Roosters game last week, even if the Roosters' execution was substantially poorer than against Tigers, Souths were very solid at holding their line and letting Roosters play out their trick shots without over-committing.
Once you have maintained your shape, it's much easier to support the halfback because the opponent hasn't dragged your players out of place. The backrower can have confidence to provide extra commitment to the space that the halfback is covering, because they themselves are being supported in the middle.
If the opposition is actually dominating your middle, then the backrowers tend to defend towards the centre and that's when your halves get isolated.
I agree with @Kazoo-Kid's analysis, although I think more emphasis needs to be made that Brooks' miss on Hoy was technically very poor, and yes the inside defenders didn't help him, but it was a poor miss regardless of any further effort he put in, and, at the time, a crucial miss.
Difficulty for the coach is we don't have that consistent defensive shape worked out yet - we are easily stretched and goaded into defensive mis-reads. We have periods where we battle and work hard for one another, but I wouldn't call it smart defence; there's a lot of scrap and small judgement errors.
You need to get the basic defensive movement down pat before you can start worrying about having specific enforcers to cover your halves.