Not sure about the reasoning here but you can't have the cake and eat it.
You can to a degree. When the inevitable next downturn occurs in the industry there will be a surplus of pilots. Historically some will avail of special leave but others may well be surplus to requirement. The unions position has historically been that jobs and pension are more important than T&Cs. In the past this has meant that in order to preserve jobs then you must hand over pay and conditions. In having contractors you can cut them adrift and the company has less of a position to argue for your hard earned.
The issue is really the numbers of contractors and the impact that this will have on the lowest pilot on the seniority list. Its a fine balance.