Punk continued on through the 80s. How can you deny Hardcore/Post-Hardcore as a continuation of punks attitude... The DC punk scene, the california hardcore scene, all of the SST bands... They all bought into punks attitude (although, as you said, not necessarily its sound).
I mean its kind of a difficult discussion to have -- when new wave started and when it first happened. I mean its hard to deny that Talking Heads and Blondie both released albums in 1977, which is pretty much the year punk happened. Then again, post-punk started in 1979, and as i'm sure you know, many punk bands adopted a more sophistcated aesethic/sound. Thats how I see it.
But, anyway, I don't really find Joy Division to be that important. Although one of my favorite bands, they simply don't have the same influence as bands like The Sex Pistols or The Clash.. they're both bands which pretty much spawned an array of genres.. where as Joy Division was simply a band that existed under a title which was very loosely defined (that title being post-punk), and created music under that genre that exists in a single time (the 1980s, and I guess against in 2000)