Never underestimate the potential for the developer community to change their minds (or the hosting company to 'add extra value' at any time).
Github may be the current darling, but I recall when it was Google Code and Freshmeat. I even remember Codeplex for all the Microsoft stuff and even that now seems to be migrating to github.
So keeping Sourceforge going, and preparing it with some much nicer navigation and website functionality would go a long way towards being ready for when github stops being cool and starts to look pretty crappy. If SF could provide the kind of fancy management portal for a product with wikis and decent bug trackers, and possibly the return of their old compile farm, it'd compete favourably with github and we'd be free of a dangerous monoculture.
So I welcome our new source control and discussion overlords, lets hope they return to their glory days.