Iggy has had his fair share of playoff blunders throughout his time in GS, you just don't notice them to the same degree because Steph, Klay and Draymond have been present to balance it out. Steph is always going to be the guy who has the ball in his hands down the stretch,
when the pressure is on. Ben won't be thrown in at the deep end like that. Because that was the problem in Philly, where he was the one and only primary offensive initiator, but he wouldn't have that responsibility on this team. He'd be free to inject himself as he pleases, which would lighten the mental burden he's had previously.
All the ball-movement and player-movement in this system would limit the times where he's caught in dead-ends. All you've got to do is look at how Looney is treated and what space he has to work with, and imagine what Ben would do in his place. Three-plus defenders worrying about chasing Steph and Klay all over the floor, and Ben would eat at the rim.
Iggy is soon to be 38-years-old, you can't expect too much out of him, especially if this team hopes to go on a deep playoff run. And with the wear and tear on Draymond's undersized body over the last near-decade, he needs another defensive anchor to help shoulder the load. That goes for playmaking too. Steph, Dray and Ben can all share the duties of running the offense; it'd give them more options and it would make the offense less predictable, which would in turn cut down on all the wasted possessions they have with turnovers.
Wiggins' defense has been overrated for a minute, and he has little-to-no concept of team defense, which the Warriors would benefit from more with Ben. Kuminga is young and can't be expected to take on that sort of role this early in his career, and by the time he has it all figured out, this core's window will have passed.
And they need more than just defense at the wing position. They need a defensive game-changer, and that's what Ben is. He's a game-changer on both ends, despite whatever flaws he has. You can't look at how he's performed in Philly and translate that to how he'd play in GS. It would be the perfect environment for him to deafen his inhibitions and take his game to the next level. It also give the Warriors a younger, bigger body to help navigate through a postseason run, which they're in need of. After all, Steph, Klay and Dray aren't young anymore.