I don't necessarily agree with their decision to put it behind a button that isn't obvious, but I can see why they made that decision. Every additional step required to set up a chat makes actually getting to a chat more tedious.

All of the encrypted messaging apps have to deal with the fundamental problem of deferring trust verification to something else, somewhere else, and that work has to go somewhere, sometime. It seems Signal has decided that (1) it shouldn't be centralized, like through Keybase, and (2) verification of identity on first contact is less of a risk than MITM, and the fact that it's possible and people do it is enough of a general deterrence. It'll stop anyone who wants to be stealthy.