You could be right, the 'tweens'/millennials could just choose to stick with SC. But I think it's just as telling with SC's recent spate of features that are moving away from the ephemeral nature of the app ( ie Memories ). Where you can store or save Snaps or Stories.... oh you mean sort of like a personal gallery? Which leads naturally to newsfeed.
Bottom line is that the photo/messaging apps are converging. And I don't think there's necessarily any loyalty to a particular platform especially if they have very similar features.
The crowd will move to the platform where their friends are, and where the celebs/bloggers/artists/influencers are. And IMO I think IG has a slight advantage with that group at this point because of the non-ephemeral nature of their app which led/leads more naturally to those types building their 'brand' and following. Nobody wants to spend hours and energy creating content that simply disappears. But these types have followed the crowd onto SC and lived by the SC rules, but I'd have to believe they would prefer the other model if they had a choice.
So I guess what I'm saying is that these 2 apps are converging to the same point, albeit from different ends of the spectrum with different pressures. And once that happens, loyalty to a platform goes out the window, and being able to capture the cool kids and influencers will be the key to where the rest of the crowd goes. So I don't think it's 'certain failure' from IG's point of view, and certainly still anyones game.