At most that will just get it dismissed against some individual plaintiffs. It won't affect their case against the city and any companies such as the engineering firm that was a defendant in this particular lawsuit.
It'll come out of their taxes, which the city will either have to raise, or cut costs somewhere to pay for. The people responsible will be neither criminally nor civilly liable, leaving zero disincentive to others.
If the engineering firm is acting as an 'agent of the state' then the protections extend to them as well. Otherwise we wouldn't be able to protect private prisons, etc from being sued, and we can't have that happening in America. So the court invented a fake protection (qualified immunity), and then invented a fake protected class called 'agents of the state'.
Which certainly won't get thrown out over qualified immunity.
(And if they win a case against the city, it's coming out of their own pockets!)