By the way, that's why we have this phenomenon: http://en.wikipedia.org/wiki/Aircraft_boneyard
a) Never take any planes out of service
b) fly every plane until it crashes
No manmade machines have infinite service life, so a is out. No airline passengers will accept b.
It's not like a car, where it can be driven until the wheels fall off. Planes are flown until they need an expensive scheduled maintenance check, even if they seem to work just fine.
(To be absolutely clear: I don't think this is a bad thing. Maintenance checks are good. Falling out of the sky is bad.)
Go look at their route schedule. Looks like penetration to me to more established areas.