One thing about Spirit is that the late-nights (Fallon, Colbert, etc.) are pretty unanimous in using them when they want to make a joke about a bad airline or what flying has become. Does Frontier want to merge into that? As the midwest saying goes, "The rocks come with the farm" and that's a big one. Even if they keep the Frontier name for the new entity or come up with some new name, if they make Spirit go away then that reputational "baggage" will still follow along.