"but there's ultimately a limit to how effective personal cars can be when population density exceeds a certain level."

And yet tha vast majority of the US is nowhere near that density. The most dense areas already have public transit. Could those areas use some additional funding? Probably. Are there other issues than just funding? Definitely. Is the title misleading and presenting a false dilemma? Yes, the US is investing in both, not just one or the other.