Every once in a while we are presented with a Doomsday scenario like this. So my point here is not take stand with one or the other just to give an overview or the possibilities the new feature can give you.
To show you a historical trend it was always full of copiers. Some were straight plagiarism when Edison copied Tesla some were really clever tweaks which is rather innovation.
There is always place to improve something but in this case Instagram just took what's working and implemented it on their own platform and even if the surface shows they haven't changed a thing...
Introducing Stories to the platform is going to make a big difference.
Obviously it won't kill Snapchat because the two platforms has vastly different user bases and ways to form content.
I mean Snapchat was always about "not so serious" content but Instagram placed really big emphasis of artistically made content. It is like a museum.
By taking Snapchat's feature Instagram creators have a lover barrier of entry. Stories brings a way to create casual content and increase interaction even more on the platform.
What do you think?