I think both Signal and WhatsApp "trust on first use" like SSH does it. The issue here is that: 1) the vast majority of users have those MITM notifications of

I think both Signal and WhatsApp "trust on first use" like SSH does it. The issue here is that: 1) the vast majority of users have those MITM notifications off by default (because WhatsApp decided it's best that way) 2) WhatsApp generates its own keys in some scenarios, like when people switch their SIM cards, so the "trust on first use" that worked on the original SIM is gone out of the window now, and the users won't even know it because the notification is off by default. Actually now that I think about it, this is why WhatsApp must have let the notifications off by default, because they knew they would generate their own keys this way, which would generate a lot of those notifications all the time.