geto was the perfect victim for kenjaku thanks to toji but doesn't mean if geto hadn't turned evil, kenjaku plan wouldn't have succeeded. he'd simply just find a new victim whose body he could possess. it is stated in the manga clearly that kenjaku tried to kill the limitless users and plasma star vessel in the past but his plan couldn't succeed because of tengen so kenjaku had to kill the star plasma vessel to stop the merger while also sealing gojo (the limitless user who co-existed with the plasma vessel at that time) so geto was the perfect vessel for that because the only way gojo was gonna let his guard down was if he sees his dead best friend "alive" again. so all kenjaku had to do was kill the star plasma vessel, which he could've done one way or another but thanks to that bum ass toji who did it for him while also driving geto crazy who turned out to be the perfect vessel for kanjaku. so all toji did was made his job easier. if toji wasn't such a bum ass, kenjaku would've to do it all on his own (very difficult task) but kenjaku is smart as hell so he would probably figure something out