Getting everyone in an office to use Signal is absolutely easier than getting everyone to use PGP. There's no specific workflow that you can do in email that you can't do in Signal - attachments, multi-party messages -they're all there.
Using a chat application at work, even one with a desktop application, is a culture shift, but I would say two things to that: first, if we take a look at Slack adoption, it's clear that many people are willing to use chat-like applications for work, and second, if the pain point is getting one's team to use new technology, then PGP is the last thing you want. Even if a tech sets everyone up so that their mail clients are good to go with PGP, anytime a user needs to email a new contact they need to:
gpg --search-keys --keyserver keys.gnupg.net email@example.com
gpg: searching for "person" from hkp server keys.gnupg.net
(1) Person < firstname.lastname@example.org >
Keys 1 of 1 for "person". Enter number(s), N)ext, or Q)uit >
After they press 1, they need to type:
gpg --fingerprint email@example.com
And then verify that that fingerprint corresponds to the publicly posted fingerprint of their correspondent, or type:
gpg --list-sigs firstname.lastname@example.org
And then check to see if anyone they know (whose key's fingerprint they have already validated) has signed "email@example.com"'s key. You can also do the equivalent of the above in one of the clunky graphical user interfaces written for PGP, but the experience is just as bad.
With Signal, you don't even have to worry about identifiers, because it uses your phone number and runs a set intersection on your contacts. The comparison with Diaspora isn't really apt; Signal's social network is already there and we're already in it: we all have phone numbers.
If the problem is getting people in one's office to all use a new technology, then PGP is a terrible solution.
Besides all of that, there are many good reasons that we should hasten email's demise in the first place. It's easily spoofable when used without PGP (so, most of the time). Unless you are sending email exclusively between accounts with large webmail providers, STARTTLS is often not enabled on the email server, which means that your email is sent entirely in the clear without even the benefit of SSL encryption at some point in its travels, even if your connection to your webmail provider is encrypted with SSL. If you are corresponding with an activist in Bahrain, whose email is:
and their mail provider is GoDaddy, then their mail server doesn't have STARTTLS. If they are using a mail client instead of webmail, then any links or attachments that you add in your email to them can be replaced with malicious ones by the Bahraini government.
Of course, you could solve this problem by using PGP, but good luck with that. I challenge anyone to successfully recruit more than 10% of their regular contacts to adopt PGP. I've tried it. It doesn't work.