Just out of curiosity, is there any limit to a commercial aircraft's lifespan assuming that new or recycled parts remain available and the owner is prepared to wear the cost? Does the manufacturer or any aviation authority specify that after a certain number of flight cycles, an airframe has had enough?
There's an airline in northern Canada that still flies radial pison aircraft from WWII like DC-3s.
As long as the Saab 340 remains cost effective to operate (and parts are still readily available), there is no reason for Rex to replace them.