Yes people will leave this server for an official one. People here fail to realize that there are many players who enjoy vanilla but do not enjoy playing on a pvp server. There is also the issue with server stability; rollbacks; boss encounters; pathing; etc etc etc. While none of this is game breaking for the vast majority of players (I assume anyway) there are people who will definitely leave.
I think the one saving grace for this server is that it will progress to the Burning Crusade, presumably before classic launches. I'm sure there will be a lot of players who stay on this server just so they can play BC. I imagine there will also be players who leave to go play Classic that may or may not come back here once they get a feel on how Classic actually plays out, or just get tired of Classic and want go relive TBC greatness.
While there isn't a subscription to play this game there are people (like me) who do spend money on xp boosts, etc to help support the folks to spend time, energy and money to keep this server going. I personally think is quite selfish for players to play for free and never bother to donate some money to keep this server alive. I think the big question is how many players does it take to keep this server going forward. If the player population gets too low I imagine that the people who run this will need to figure out for themselves if the effort is worth the reward.
I, for one, will be leaving. While I've enjoyed playing vanilla, I do not enjoy spending the very limited free time I have to play a video game constantly being ganked. It is not enjoyable for me. And TBC zones are a lot smaller than classic zones so I imagine that the pvp will only increase in Outland.