Or you can just forget the mists? This is just a regular place ruled over by a vampire. The people generally don't leave because most people don't seek opportunity elsewhere (that's a pretty modern thing). The PCs can leave any time they want, but with the knowledge that they've left the poor locals to their fate if they don't deal with Strahd.
That could certainly be a possibility, but that wouldn't change the fact that [MENTION=12731]CapnZapp[/MENTION] would still need personal PC quests to inspire them to stay within Barovia. Since he wants to actually run CoS but doesn't want to aim the end game only towards killing Strahd... he's really wanting ideas that connect personal PC quests to other ways to leave so that they have a double reason to undertake those quests (both because they are personal and important to the PC, and also because that's their way of leaving.)