Until DOJ approval, the airlines can't merge anything, and in fact JB won't even be able to make the purchase, let alone have ownership of Spirit. That's 12-24 months away. Until then they are 99.9% separate companies, other than any MUTUAL agreement between the two managements of how to manage the separate companies. As such, both companies will continue to grow and compete as if they are separate, because they are. It is also possible that the DOJ never approves the transaction, which is why both companies will continue to grow and compete individually.

It is also in JB's long term interest for Spirit to grow and ideally manage a good airline.