I totally get you observation about losing out on authenticity (fantastic observation btw!), Snapchat’s big draw is life as it happens (#nofilter). By being able to upload videos that have been produced, IG Stories might not tap into the Snapchat feeling.
I think Snapchat is so sticky because it very closely mirrors real life. My life never looks like an Instagram photo and that’s why I love Snapchat (also potentially IG Stories). It doesn’t have to for creating content!