While new/additional bus service may or may not be cheaper to construct and operate than a streetcar line, buses do not attract the development or riders, especially ones who have a choice and can afford a vehicle and parking, that a streetcar does. Like it or not (and I'm not saying I do), one of the reasons for a streetcar line is development.

And I do not see how a streetcar ride can be as expensive as a taxi ride, unless you are requiring the construction costs to be paid for in a short amount of time. This is unfair considering that the infrastructure of tracks, stations and power can last a long time. Also, electric-powered vehicles generally outlast diesel ones and the same should go for rail-based vehicles over road-based ones.