So why change that brand marketing/modelling to Jetstar
Exact same reason as they did it to mainline. If they were prepared to cannibalise mainlines branding and reputation as a premium product, what makes you think they wouldn't do it to the prop operations?
First of all it will 6-12 months of heavy loss making in NZ.
Then they'll move a few across to Australia "but they won't compete with existing routes, just leisure destinations".
Then they'll say how QLink are losing too much money.
Then JetstarLink will come along as the saviour and, lo and behold, start operating on QLink routes.
Eventually, Jetstar will be doing the majority of the prop flying and cadets will be lining up to pay money for a job, and people will be buying dash8 endorsements to jump the queue.
Well that's how I imagine it might go. Time will tell.