There are few limits to what one can promise in a contract, but I have zero sympathy for idiots who agree to a contract where a party promises things that they cannot reasonably deliver. Any damn fool who goes along with that deserves to get stiffed as an object lesson to the other fools in society thinking about doing the same thing because someone promised them the moon.
Any employee who agrees to a contract without doing due diligence on the terms deserves whatever they get. The unions got greedy and did not look too closely or think too hard about what they were agreeing to.
Putting aside the question if an average worker can predict the future or be an expert on economy, are you suggesting that the employers KNOWINGLY made false promises?