Sparrow is still getting small updates on both iOS and OS X. Mutt hasn't been updated in a few years. I'm not sure one is more dead than the other.

Also, one app became loved because it has a really innovative UI and the other is an app that eschews a modern UI. I'm not sure many people will logically make the leap from Sparrow to Mutt. More likely, people will gravitate back to to gmail.com, which should be getting a lot of improvements from the Sparrow team in the future.