Anonymous D joined in and replied with this 5 days ago, 25 minutes later, 3 hours after the original post[^][v]#1,413,470
I mean "back in the day" wallpapers (much like ringtones) were things some people might actually pay money for, so wallpaper threads were a bit more interesting 2005-2010. Sure there were sites offering free wallpapers, but now there are billions of them in the highest qualities. But yeah I guess there's also the fact that fewer people sit behind a desktop nowadays, too.
Anyway there are still wallpaper threads on 4chan, hell there's an entire board and it's not ultra dead: https://boards.4chan.org/wg/2