Some unions have successfully negotiated collective bargaining agreements that constrain or disincentivize the behavior you describe ("respond to small changes

Some unions have successfully negotiated collective bargaining agreements that constrain or disincentivize the behavior you describe ("respond to small changes in the macroeconomy by instantly throwing thousands of people onto the street"). One of the most common is a mandatory base severance payment (e.g. twelve weeks of of base pay) plus a certain amount for every month of employee tenure (e.g. four weeks of additional pay for every year at the company up to some limit). This gives the employer an incentive to consider a longer time horizon when hiring.