If your app is not popular with terrorists and child rapists, it is not secure.

I doubt that is 100% true in either direction. People, especially non geeks can be duped into thinking something is secure

If an app is popular only with terrorists and child rapists, it's a weird choice for a business.

Militaries, for one. https://wickr.com/secure-communication-for-the-military/

I know Australia's Ex-Prime Minister, Malcolm Turnbull, used Wickr while he was in office.[0]

[0] https://www.abc.net.au/news/2015-03-03/malcolm-turnbull-uses...

Drug dealers are one thing, but this just goes to show some truly awful people are using it