You're absolutely correct. I'm not arguing whether it's possible to use Instagram like one would use Snapchat — it is, and my points won't make any sense from that POV.

I was laying down my observations on what's the default, and hence seemingly most common behavior for the given network. All snapchat accounts are private by default (yes, you can make stories public like on Instagram). All Instagram accounts are public by default (yes, you can make it private) The power of default has a significant role to play in how people think about and use a particular network. This is even more true for the people who are not very tech savvy, which is the majority.

Again, not arguing if Instagram is better than Snapchat, or vice versa. Instagram probably will win the majority of the market, and will have an order of magnitude, if not more, users, engagement and revenue. Will Snapchat become irrelevant and die? I'd say it's too early to make that call.