The cost of the trades is the hidden cost in this equation for mine. If Preuss does well and Gawn is overpriced but still the clear top two he's been for a while now, you're still probably looking at 2.5 trades to close that position. Depending on the value you put on a trade this really can move the needle one way or the other.
Rucks are so difficult because you don't have alternatives (barring something significant). You have two players and if you don't have them you're leaking a lot of points. All the other positions you can always find value, every year there are a couple of guys who move into the midfield mid season due to an injury, someone who has an early injury roll through for 3 weeks and are dirt cheap. Obviously it's always possible it happens to the rucks but it's a big ask.