Why merge with Spirit when AK is ready to give up 63 airbuses (+28 new orders) with an average age of 7 years? It may be coincidental but UAL's and AK's (merged) fleet plan are both due out about the same time, end of 2017.

https://www.fool.com/investing/2017/04/02/alaska-air-will-keep-virgin-americas-airbus-fleet.aspx