How exactly would Shen have destroyed kung fu? Just by killing the Furious Five? Okay, but they're not the only masters of kung fu out there. There were probably millions of students who already learned it. How was he going to find and kill every single one of them?
shareI think the idea was that the cannons would make kung fu obsolete.
shareKung fu would be destroyed because Shen's weapons are so much deadlier and anyone could use or obtain them where you have to learn and practice kung fu to be good at it.
The rhino was the perfect example, he was a better fighter then Shen and was very powerful, but Shen knew this and even says so, and then lets his weapon kill a master of kung fu. The point being Shen's weapon is easy to use and even a great fighter can and probably will fall to it.
"Always two there are , a master and an apprentice"
Kwai Chang Caine would have come along during his wanderings of the earth and saved all.