The thing was that France and Britain actually engaged in a shooting war during the American Revolution. Without the direct military support of the French, Lafayette's efforts would likely have been in vain. It was the French Navy that bottled up Corwallis and Yorktown and forced the surrender that ended the war.

To be honest, the reality is that Hong Kong is just not worth the costs of a US-Chinese military conflict. Given the Chinese view of history and how they view the partition of China among the European powers in the 19th century, this is likely a red-line issue for them that they are willing to go all out on. For good reason, no other country sees it that way.