Politics or not, I have to believe that if a carrier the size of UAL or AA downsized in ORD to a significant degree, that would open the door for JBLU or any number of other carriers.
I think its more a matter of scale than anything else. If you want to operate less than 10 flights a day, for example, you're going to have a LOT less pull than if you propose operating 100.
Then, consider what could happen if a MAJOR user of an airport facility suddenly reduces a large amount of service. That kicks the door open for a carrier like JBLU to implement service.
You have to give credit where credit is due: JBLU has demonstrated the kind of flexible management that allows them to pull out of bad markets in favor of more promising ones.