Facebook's strategy certainly has worked so far, but I doubt it's sustainable. Every time Instagram copies a Snapchat feature it is a pivot from their original vision. Unless Snapchat starts copying Instagram, over time the two will diverge, one is a focused and coherent product, the other becomes a feature laden beast.
Also internally I can't imagine how the Instagram engineers feel about their roadmap being "continue to copy and shoehorn Snapchat features into our app".
Also internally I can't imagine how the Instagram engineers feel about their roadmap being "continue to copy and shoehorn Snapchat features into our app".