While the strong brand and loyal customer base make a full closure unlikely, the pool of investors who will be able to pull the struggling airline back on track is limited — and, now that Spirit has rejected a merger offer from Frontier ( FRON) for a second time, got even smaller.

Spirit Airlines is rebuffing the latest takeover offer from Frontier Group, with the discount airline saying that it prefers focusing on its current reorganization plan.

Spirit Airlines rejected Frontier Group's acquisition offer worth about $2.16 billion, marking the latest failed deal in the years since the companies began discussing a merger.