Going by your logic though, what would stop a Five Eyes country like the UK from pressuring the developer of SimpleX into creating a backdoor? Besides, as discussed, even if it were bulletproof, it's improbable that the victim would have no other apps on their device, one of which could be exploited by the likes of NSO Group. The creators of Android and iOS are also obviously US-based, so your point would have to apply to them as well. From there, if someone remotely gains full access to the device, it won't matter if you use Signal, Telegram, WhatsApp, SimpleX, or that new Russian thing. However, having e2ee is still better than nothing in that it protects from other attack vectors, like the ISP analyzing the traffic and reporting to the government.