The 'voluntary severence' package being offered is about half that offered by quality companies like M&S. It's 'take the new contract' or 'we don't promise anything about the future' like promotion, base change and so on.
So, in short the new employer is not interested in maintaining /honouring contracts as required under law because it 'doesn't fit in with their business model'. The model is, in year one, to pay all staff and contract companies slightly less the going rate, charge passengers slightly less than the competition and fill the planes. In year two you tell staff and contractors that times are hard and that you can only keep the contract if you accept less than last year etc. Bonuses are then paid to top managers. No guarantee for the shareholders who after ten years haven't seen a single dividend. But buying competitors (as opposed to just being better and winning passengers while expanding) and changing staff contracts is apparently part of the business model.
Obviously anybody who wants to challenge this is welcome to go to court. Employees will need deep pockets to counter the expensive barristers employed by the company.