> What's the benefit of decentralization? Not being snarky, I just don't really see it. What does a decentralized PGP email have that I don't have with my Signal Messenger?
It's a lot harder to block. You can have anyone run a mail server on any port (SSLed if necessary), which means you can use it for secure communications inside any "great firewall" (like that of China or Kazakhstan), or even in a country/region that's been cut off from the Internet (which we've seen happen for various amounts of time in Syria, Crimea, Turkey...). WhatsApp/Signal have a very limited version of this by getting makers of popular HTTPS services to work with them, but those services are subject to their own commercial pressures (all the big names have been known to collaborate with e.g. the Chinese authorities in the past) and attackers always have the option of just blocking those services outright.
It rules out a whole class of attacks involving compromising the central servers and tricking the client into redoing an initial exchange. If the Signal client is implemented correctly this shouldn't be an issue, but it's an extra attack surface that simply doesn't exist for OpenPGP.
Also compared to Signal et al it's more practical (though still difficult) to use pseudonymously, because a pseudonymous email address is eaiser than a pseudonymous phone number. Signal advocates talk about having deniability because your messages aren't provably signed by you, but I don't think that advantage exists in any reasonable threat model - if we're worried about a state adversary hunting down everyone who corresponded with dissident x, they'll find it easier to track down a phone number than an email address, and once they've tracked down one of x's correspondents (i.e. someone who has a phone with a number that exchanged messages with x) they're not going to be bothered by the fact that they don't have signed mathematical proof that this is the same person who corresponded with x.
> Signal was designed from the ground up with UX in mind, and made several important trade offs for it
Such as? At the protocol level I mean.