You're right that it may only measurably improve the lives of a few hundred - but most of the cost is upfront; There'll only be small maintenance costs on the bridge for the next 100 years until it needs to be replaced. Even then, the existing infrastructure should lower the cost of replacement. 

Without projects like this, how else is the city supposed to build out the infrastructure needed for the future? I can't think of a city project that would be totally fair in terms of improving the lives of all residents simultaneously, can you? If our public spaces are merely the sum of their parts, then how could any individual project be considered totally fair? if there's no way for a project to be totally fair, then does that mean the city should not commission any projects?