Popping in -- I think you likely have nothing to worry about. Dreamwidth makes it very easy to crosspost, so it's all the more likely people will keep their journals here (though some have been blocking comments). Also, it's very easy for people set up RSS feeds to Dreamwidth journals, so those not crossposting can just show up on your flist with just the click of a subscription button.

So in the unlikely event that all of fandom migrates there in the next 3 years, it will be easy to stay in touch.