It costs a lot less to build transit infrastructure before or at the same time as everything else compared with adding it later, even if the line is underused as density is added.

The best alternative is a well-planned phased line with carefully protected right-of-way and a dedicated source of long term funding. Bonus points for it being a combination of value capture taxes and the transit agency being a property developer in their own right around stations. The early phase can be inside the boundary of current development so there are people to ride right away. Developers can build and market using the upcoming line, and prospective residents can be confident it will happen with funding secured.