Baseless that he's the final villain
And no, Shonen's always have progressively stronger antagonists, it's literally like one of the defining traits of a Shonen, there aren't any stakes if the next antagonist is weaker than the last, it's why Shonen are notorious for power cliffing, because they need to make the villains stronger and stronger and stronger for it to make sense.
Kaido was a raid boss who required asspulls to be defeated, and yet Imu or whoever is next will obviously be stronger than Kaido.