Although you made some interesting points, like the point about the hostility of the interface. One of the reasons you don’t think Snapchat is going to grow is one of it’s core factors leading to success. The intimacy of adding someone on Snapchat provides a somewhat genuine feeling which isn’t really created by any other social media.
To address the point of adding celebrities or skilled artists like you can on Instagram usually a quick Google search provides you with the public Snapchat they have setup where you will find the exact same content they post on their Instagram stories.
Coming from a student, no one on campus is using Instagram stories for anything they wouldn’t put on their snap stories. The sensitive information that sometimes gets posted on the stories, or sent back and forth between groups, is the currency which retains users that Instagram does not have.