The Qantas Freight (not AAE for several years now) contract would likely end as soon as Badgerys Creek is up and running. The contract exists because of the Sydney curfew dispensation given to the BAe146. That dispensation ends when the other airport is built.
Based on Badgerys Creek progress to date (40 years of talking and no action) I think the 146 will be around for quite a while yet.