We didn't even get to see ichigos bankai in action the first time because Yhwach broke it. He acknowledged its power and decided he needed to break it in order to win. Yhwach really proved ichigos strength here, he even said earlier that ichigos power is so formidable he had no room for errors. This acknowledgement from the most powerful person in the series puts ichigo above kenpachi. Kenpachi got stuck with an underling who he had to fight with 2 other captains. At this stage, ichgo really is the most powerful good guy. If he wasn't then why was the fate of the entire world put on him to defeat Yhwach and not kenpachi??
Another thing to note, ichigo, aizen, and ishida didn't beat Yhwach through strength. Subbing kenpachi out for ichigo would have resulted in a loss. They won because zangetsu helped ichigo get the last blow in despite Yhwach breaking ichigos bankai again. If it was kenpachi there would not have been a second sword there to deliver the final blow.