Rim protection numbers like FG% at the rim only refer to that, FG% at the rim.
You can have weak rim protection once opposing players get to the rim, it doesn't necessarily have an outsize impact on your defense if opposing players struggle to get to the rim. A lack of deterrence at the rim certainly weakens the defense, but it is not a death sentence.
Currently, Towns has a defensive EPM rating of -0.2, which suggests he has had a mildly negative impact defensively. Which of course is far from ideal, especially from a center. He's third-worst on the team after Brunson and Sims. So, not a complete liability, but not good either.
But we always knew that Towns wasn't a rim protector. It's just not part of his skill set.
Which is why this defense will only function if our perimeter defense tightens up. We know Brunson's a liability despite his commitment because of his size, so we need Bridges to step up and play strong defense, which he hasn't done yet. OG's doing his part, so Mikal and Hart are the two variables that will decide how our defense performs moving forward.
So long as KAT plays the 5, we need to prevent dribble penetration or easy rolls to the basket from the perimeter. The answers are schematic but also rely on defensive commitment from the players. I hope we can see some progress before defaulting to Precious or Mitch at the 5.