If Jetstar really wants to keep them for the long term then the conditions will need to improve (unlikely with inadequate pilot industrial cohesion) or they'll need to contract them in for a longer period.
What would be the mechanism to bind a cadet to any operator? The
Jetstar Cadet Booklet (V4) is very clear on the funding, it is up to the cadet to self-fund along with the Federal governments FEE-HELP program. Jetstar doesn't appear to put anything at stake - it has no skin in the game.
Traditionally, the binding mechanisms have been bonding for type course along with the more subtle denial of ICUS time, therefore preventing ATPL qualification. Close to command upgrade time, ICUS is then allowed to be logged, thus allowing the necessary ATPL.
Without the operator actually signing a contract to fund part or all cadet training, the self-funded cadet is a free agent.
Log book time is solely & irrevocable the property of the pilot,
independent of the operator. I believe this was a the logic behind the now defunct MPL licensing system, where logged time only had value within an MPL operator's check & training system as I understand it (corrections please), and was not readily transferable to another operator.
As to increasing T&C, with or without industrial cohesion, Ryanair have demonstrated that improvements will occur when there are significant flight cancellations. As they say in commodities, the solution to low prices is low prices! The market ultimately will adjust, although the price discovery efficiency of the pilot market is low as the market power has largely been with a small number of operators, compared to the large number of individual pilots.
I would expect, just like Ryanair that there will be a discontinuity, ie no change, then a sudden significant change in pay in a single move. Something along the lines of a loyalty or incentive bonus, conditional on an extended period of service. The underlying contract may not alter much as operators do not want to lock in large contract increase future liabilities.