Lodown, seniority guarantees a supply of pilots when the time comes for expansion. When you join a seniority system, your time in the company should be money in the bank for both the company and the pilots.
Jetstar may think they are gaining flexibility by doing this, but in truth, who is now going to consider Jetstar as an option for that first jet job, when your wait in the orderly queue can be held up, entirely at the whim of the company?
Jetstar are screwing themselves as well as their own pilots. In a time of pilot shortage, you cannot afford to pi$$ off pilots who have invested parts of their careers in your company.
You need FO's to fly aircraft too, or have they forgotten that?