You misunderstand, they are talking specifically about encrypted communication for the new use case. Think of it instead as trying to replace email for unencrypted communication, you need a different use case as the user base is too entrenched. In contrast GPG encrypted messages are a small community.
The fact that GPG is used for other things is fine, as it will still exist and can still be used for those things.
Every Linux distribution signs packages with it. So that's at least 100M users right there. Several APIs regarding domain name and AS registrations is based on it. I know at least two financial systems that signs their messages with it. That's just mentioning a few I can think of right now. I'm sure it secures a lot lot more. It's used all over the place and it seems to do the job fairly well in most cases.
Is it suitable for signing email in 2015? I don't know. Things could certainly be improved. I know there are more suitable schemes for IM communciation. But the software and the underlying protocol is not something you can break in a heartbeat. It's important software.
Perhaps we need to speak more of the evolving use case in email?