The difference is, with the App Store there is policies against malware. Furthermore, it's harder to hide such an attack as you can't press a button to deploy a

The difference is, with the App Store there is policies against malware. Furthermore, it's harder to hide such an attack as you can't press a button to deploy a special version of the client just to Joe Doe whose private pictures you are interested in. You either have to deploy it to a large group, or not at all. With js, you have full control over the HTTP server and can give that specific person a specific version of the client. Yes, the updates are automated on mobile devices, but not under total and full control of the app developer, unlike for websites. And people do monitor what e.g. the Signal app is doing, or WhatsApp. I don't agree though that all web-based cryptography is snake oil though. Targeted attacks from the site operator are not the only thing you want to defend from. Say the site operator is asked by authorities to hand over data they store of a specific user. Then the insecure web based cryptography is enough to protect the data: everything that's stored is encrypted.