IBxpress and Vueling keep getting fatter on IB's flights while IB gives them technical maintenance,handling and leased aircraft at rock bottom prices.
Let me think about this from another angle.
Iberia, who are making massive losses, would need to have scaled back it's flights and would therefore have too many aircraft. These excess aircraft have been leased out rather than being parked somewhere. It doesn't matter who they were leased out to.
Iberia Express and Vueling are getting fatter because they're leaner and have a lower cost base, so can make a profit operating the same routes. Iberia get the income from maintenance, which helps to pay for a large fixed cost. Presumably the handling income is at least 'at cost' or better still at a profit, so that helps Iberia also.
There's no such thing as "Iberia's flights". Everything airline has to compete with other airlines and no operator has a divine right to its network.
If Iberia Express and Vueling weren't there someone else would. Perhaps Air Europa, Ryanair or Easyjet. It's makes no difference to the fact Iberia was ripe for restructuring in order to stand a chance to survive in the long term.
The medicine is hard to swallow and I really feel for the employees, but there is no other way forward than to create a new leaner Iberia.