We have a lot of fairly new 320s (and older 319s) leaving our fleet as new 321s and 320neos arrive. Indigo could easily transfer some of these airframes to one of their other operations, which makes sense for Indigo to be able to do.
New airplanes are expensive and Indigo is contractually required to accept a significant amount of new aircraft. Too much debt too quickly can be detrimental to many businesses. I am no business or management expert, but it occurs to me that it could even be in Frontier's best interest if Indigo did have the option or ability to transfer some of that risk (and debt) to another operation if it was in Frontier's best interest.
I am trying to stay positive and must admit there are many times when I am skeptical about Frontier's future when I see and hear of some of the decisions that Indigo makes (many of which seem to only benefit the short term bottom line and not Frontier's long term best interest).