I think you're on to something with regards to subways in urban environments moving a number of people that a highway expansion would struggle to match. Feels to me like movement of people should be the real metric and, given that, that we should be prioritizing transit investments in places like NYC, San Francisco, and West LA (as opposed to light rail to Azusa).

Downs-Thomson Paradox is also interesting here and relates to your second paragraph. In a built out environment, transit trips and car trips will take roughly the same amount of time, door to door. Transit advocates have used that finding as a way to complain about road projects, but I think it actually is a much more interesting finding that suggests a more multi-modal approach to planning. I wonder how relevant or irrelevant it is for non-built-out environments.

In the US at least, really the complaint from transit advocates is that funding is and continues to be overwhelmingly road-centric, with a small share for transit and basically a pittance for walking and biking.

Usually the issue with suburban transit investment occurring over urban ones is that urban areas are so weak they cannot fund expansions by themselves, and if you want the suburbs to pony up they’ll get their pound of flesh