Economically 'safe' is the key, especially when you don't have control of the stadium or its costs!!
As stated by others, Poole still get amongst the better attendances across either league because fans (mostly) like what they see and keep coming. Danny Ford has only recently stated (Speedway Star) that Poole will not replace Birmingham next year, nor any other team considering stepping down (eg, rumours about Spires and Tigers!!). Also if/when Coventry come back Matt Ford (as the Promoter) has said it would be at Championship level.