I know this is kinda a trap, but Harry didn't keep Sue going because Jum Butcher knew the story was about done. Players don't usually get that knowledge, so they like to pad their bets. Plus I think your pushing your argument pretty far, to compare a enchanted duster to Sue. Even if its at the level of Mystic Toughness... again, its only 3 armor, and six more boxes of physical stress. Tough yes, but far from impossible to defeat. Its not the "I win" button so many seem to fear.
Ok, maybe Sue comes really close to a "I win" button but I got a soft spot for the old girl.
Ways and Means is the one who compared Sue to the Item of Power version of the duster, not me.
"Only" Armor:3 and six extra stress boxes?
"Only"? I fail to see how more than doubling the stress track and having reliable, always-on, unlimited use of armor good enough to stop rifle rounds is "only" anything.
Mythic Toughness means in a physical confrontation, you need a 14 shift hit just to force a
mild consequence (assuming Endurance is at 3 or above). That means even with Mythic Strength and a decent weapon, you'll need to beat the defense roll by 5 or more just to barely scratch them. At a Submerged game, even a wizard would have a whole lot of difficulty throwing something at them that will even get their attention. At lower refresh levels, the problem's even worse.
Absent the Catch, physical confrontation has one hell of a lot of bite taken out of it. It's not an "I win" button, but it's pretty damn close to "I won't lose, or even be barely injured."