If you think oakmont’s only defense is the rough, you’d be wrong. Its greens, its length, its topography, its 300 bunkers, and its ditches provide the bulk of the defense.
As for players shooting -30 if the rough was mown at 2.5”, all the National championships they’ve ever hosted, only 27 people TOTAL finished under par. JJ spaun made it 28 because he sunk an 60’ bomb on the final hole. Out of ~1500 people. Im not proposing lowering the rough to 2.5”, but even then the scores wouldn’t sniff -30. That’s laughable. With the rain they’ve had this spring, mowing it at 4” would’ve been an ample test, might’ve given guys a couple more reasons to try a heroic recovery, and the winning score might be -3 or -4. Or maybe not.