> Paved streets could be added much more quickly than streetcar lines

Streetcars tracks can be put on the ... street. The hint is in the name, kind of. :) . They're also much lighter than railroad or metro tracks. You can have level crossings and tight corners.

I don't see them as especially hard to build. Cars probably require a lot more space, when you add all the multiple lanes, intersections, overpasses etc, when you take into account their low density.

So I wouldn't see it as obviously going either way.