I think the server will live on for quite a while after Naxx is cleared. The only issue I see longterm is new players becoming increasingly frustrated playing against T3-geared players in battlegrounds and world pvp.
Imbaheals <Blacklisted Raid Leader> Local Horde Kidnapper of Tarren Mill. Rift Allstar Cleric 2011-2013 World 1st and 2nds with guilds Addiction/Blackout/Babytown/Trinity. https://www.youtube.com/user/imbaslap/videos