I've used GPG since... I don't even know, for a very long time. However, since I communicate a lot internationally, and I don't know (and I don't want to know) about every country's regulations regarding encryption, I gave up sending encrypted e-mails at the very beginning, but I still always sign my mails. I never even thought about how many people use or don't use GPG, it's just been there, ever so useful - and I think that's good so. I think "run its course" is harsh though. Why? Because one Moxie Marlinspike says so? Bollocks. If it's useful - and it is -, it's good to have it.
Not only that, but look at the Enigmail interface. Once it's installed and configured, it's only clicking the icons in the status bar and entering a password. I sign all mail as well.