Of course I don't have any concrete proof. If there was concrete proof we shouldn't be having this conversion. My main issue is that it's centralized and that's a huge black box. People obsess with this "but it's protocol open source" like headless chickens when that's not the issue. Open source is like the step one when it comes to private and secure messaging. It just comes down to if you trust the devs and those doing the hosting. When it's central all of that thrust rests on that one group and their hosting service not fucking you over even if they can or can not read the encrypted messages themselves. I'm not concerned signal keeping people's dickpicks private here in that that even whatsapp is as good as any.

I see I made the mistake of coming to an obvious fangirl meeting to have an serious discussion about security merits.

Even if we assume that man in the middle attack is impossible with signal. Intelligence agencies care more about metadata anyway. Remember that getting meaning from terabytes of daily messages hasn't really been viable way to mass spy anybody until very recently, since you needed humans to read them individually to get any wider sense of chat logs. if they know who talked to who and when. With those they can social graphs and get a list of suspects when everybody is tied to an identifiable phone number. Yeah they won't directly get incriminating chat of somebody ordering drugs, but they can go nab the dealer and their associates with that info. Or they can have a group of key activists followed if they know that when messages between these people spike just before a protest happens.