Changing a lot overnight is one of the main reasons JB is pursuing Spirit. They want an immediate and significant diversification of their current route structure. Thinking they're going to acquire Spirit only to shove them all into BOS/JFK/EWR because they're a "NE airline" is an asinine take. So no, it's not valid to worry about them changing their structure. They're going to expand their current structure. Will there be growing pains? Of course, but this has been done dozens of time before. I'm sure they'll be just fine and that most of NK's hubs will remain.