"There are perennial claims that consumers don't want privacy"
There's perennial evidence that consumers won't act toward privacy except outside of very, narrow constraints. You're analysis of Instagram to Snapchat is correct. Yet, with the carrot of ephemeral messaging, they went to a centralized service who could keep all their stuff to sell to whoever with no security or assurances in the whole process. Similar with WhatApp in terms of assurances. That's the best we're seeing. Whereas, the usable, truly-private apps that jump into any of these markets get virtually no adoption.
What you're seeing is the power of marketing rather than delivering privacy. Two very different things. Kind of like how many juices market themselves as healthy and natural while actually being a pile of sugar that might harm you if you consume enough. But people drink that 100% juice kool-aid. We've also seen where that went in the corporate space where virtually nothing is actually secure despite marketing claims and the fact that corporations try to hire people that can make an informed decision about that.
Definitely money to be made pressing that button of "ephemeral" on devices that couldn't be that if you tried. Doesn't inspire a lot of confidence for those of us pushing or developing actual offerings that are secure or private. I think more can be done and even maybe widespread but the evidence indicates market will always be dominated by scheming oligopolies of evil companies pretending to protect people with their offerings. Because people don't do better as users or buyers.