There's suspense of belief because of show-producing errors/inability to completely conform to artificial reality, and there's glaring stupid plot decisions. While I'm not saying TWD is automatically a bad series for this issue (it does create tension, drama, etc.), it takes a heavy dose of suspension to say 'sure, the group never made a backup plan, seems legit'.
It's not like they hadn't all been forced to relocate several times before. Hell, just using Woodbury as a well known fallback position would have been smart.