| This answer is opinion-based.|
Due to lack of canonical information in the franchise, there is no definite answer to this question.
Shen likely meant to make kung fu obsolete to prove his cannon was more powerful so that he could rule China with no one to stop him.
When he first heard about his prophesied defeat from the Soothsayer, Shen quickly sought to change his fate and went to great lengths to do so. From this, it is implied that he feels threatened by any other power stronger or greater than him. He likely felt this way about kung fu, which is why he sought to destroy it and replace it with his own power from his weapon and armies.