If you ever installed a Debian package then you did. A long-term identity as "Bob Jones" might not be terribly useful - but that's not the kind of long-term identity we care about a lot in real life either. A long-term identity as "Debian release manager" or "Signatory on bank account xyz" or even "Wikileaks committee member" is a lot more important, and for those cases PGP becomes very useful.
> Then, there's the UX problem. Easy crippling mistakes. Messy keyserver listings from years ago. "I can't read this email on my phone". "Or on the laptop, I left the keys I never use on the other machine".
These are real problems. We should fix them. But we don't need a new crypto standard to do so! It never fails to amaze me how many people/organizations are like "I don't have the time/money/patience to write a high-quality OpenPGP libary (or a high-quality GPG frontend), but I'm perfectly placed to create a new cryptosystem from scratch."
> Your average adversary probably can't MitM Twitter DMs (which means you can use them to exchange fingerprints opportunistically, while still protecting your privacy). The Mossad will do Mossad things to your machine, whatever key you use.
This is pets vs cattle in the opposite direction. Can Mossad Mossad you personally? Yes, if you're a big enough target, but they can't Mossad everyone. Whereas the NSA can MitM key fingerprints exchanged via Twitter on an industrial scale.
> Mostly I'll use Signal or WhatsApp, which offer vastly better endpoint security on iOS, ephemerality, and smoother key rotation.
If you're using iOS you've already given up against state-level attackers. Anything actually encrypted (e.g. IRC with SSL) is more than adequate in that case. Most people don't need the jump up to PGP, sure. But it's important that the option is there for people that really do need it. It bears repeating that we know, from their complaints in leaked emails, that the NSA can't break PGP when used correctly. That's an extremely strong seal of approval for the most critical use cases for encryption.