I am on the older end of Gen Z and have noticed the opposite. Snap was cool from 2014 until 2019 but now Insta has a slight edge. I still use both but it varies by whom I'm talking to: some friends I talk to on Snap and some I do on Insta.
It's similar to how Bruce Springsteen and The Rolling Stones aren't cool, but still sell out stadiums.
Facebook (the company) will probably settle into a stable pattern like the BBC's radio lineup, where listeners roll from Radio 1, to Radio 2, to Radio 4 as they age.
I think there's so many ways to connect on the internet that every social app has it's own role and purpose. Everything is fading away as users age and their preferences change. On top of that there are always new apps which are trying to attract new users and lock them in until something new doesn't come out that's why Zuck was obsessed with competition. He tried buying Twitter and Snapchat but they refused on the other hand Instagram and WhatsApp gave in.
Snap is still wildly popular with Gen Z. The use case for them is mostly messaging and private Stories. Instagram is more of a consumption platform for them at this point. They serve two different use cases.
