It's not like kids/teens can delete Instagram either. Communication is done with iMessage and Instagram DMs => deleting them not good, especially if you don't have an iPhone. I wish there was an open platform that was encrypted by default, and isn't primarily advertised as privacy centric. If it's advertised that way then people will be scared of being seen as a terrorist or something.
Wire mostly fits the bill except it's app feels like hangouts (ugh), and it's focused on privacy, so teens don't want to use it.
Also iMessage is encrypted, but not cross platform.