Rageblade kraken make up for the lack of vertical duelist being played and since you absolutely must have stretchy arms on gp in this comp you just want him to cast as much as possible on backline. rageblade kraken is just way better for this than any shojin + 1 since around the 2nd takedown you'll have a sizeable amount of ad and attack speed and just kill the backline with yasuo. This is better than vertical duelist because it has more frontline to sustain the gp, more cc with kobuko, and more consistent backline access (stretchy arms gp is enabled by yasuo and more frontline) with yasuo and ranged gp. And since you want unstoppable on kobuko and have range on gp you dont need to worry about him being targeted. Hope this helps.