As when Knuth stopped releasing new versions of TeX because it reached a point where it was "done", software that has been around for long enough can eventually mature to the point where there's nothing left to add without expanding the scope of the project beyond its original definition.
Mutt may simply be at that point. I use Mutt as my primary MUA in a very similar manner to the author of the article, and I can't really say that there are any features that I want that it lacks. Of course, I can't speak for everyone on that point; for instance, some people actually want to send and receive HTML email, and Mutt can't easily deal with that. But for some of us, lack of HTML support is itself a feature, and when you really have to, piping a text/html MIME part to a browser for rendering is really easy to accomplish.
I know essentially nothing about Sparrow, but I suspect that it is a much newer product than Mutt, has had far less time to mature, and as such simply requires a more frequent release schedule than a project with Mutt's long history.
Basically, a comparison between most recent release dates doesn't necessarily mean what you think it means.
I knew if I made this joke I'd get a wall of text explaining why I was wrong; thanks, HN — you delivered!
On a more serious note, I use Sparrow every day. About the only quibble I have with it is some of the icons aren't retina versions (hardly a dealbreaker). Otherwise I'd say it meets my needs very well; doesn't that also make it ‘done’, rather than ‘effectively dead’?
Mutt has plenty of unfixed bugs, just look at the bug tracker and distro-specific patches (in Debian for instance). It is a wonderful piece of software, I use it daily, but its development definitely miss momentum.
Mutt may simply be at that point. I use Mutt as my primary MUA in a very similar manner to the author of the article, and I can't really say that there are any features that I want that it lacks. Of course, I can't speak for everyone on that point; for instance, some people actually want to send and receive HTML email, and Mutt can't easily deal with that. But for some of us, lack of HTML support is itself a feature, and when you really have to, piping a text/html MIME part to a browser for rendering is really easy to accomplish.
I know essentially nothing about Sparrow, but I suspect that it is a much newer product than Mutt, has had far less time to mature, and as such simply requires a more frequent release schedule than a project with Mutt's long history.
Basically, a comparison between most recent release dates doesn't necessarily mean what you think it means.