Popularity downturn is probably due to the Global Economic crisis we are all suffering, i mean not everyone has £310 to throw at a box on a whim and less will want to upgrade unless it makes you a cup of tea
As for your question on XBMC i would imagine if Vu+ keep there promise the Duo2 could be the last Linux receiver as Marusys the main arm has the prismcube coming but who knows when. The prismcube is next gen stuff and should wipe everyone else out when it finally emerges.