Public transit should never be treated as an investment that should produce a return

It absolutely needs to produce a return. There are real lives involved in building and operating transit. Without returns, that is a life wasted that could have been used to do something useful for society.

It should be cheap or free.

Quite possibly. The returns need not come from fares. The fare is for rider management – ensuring that the buses and trains aren't overcrowded. The fare needs to only be as high as is required to ensure that there isn't someone not able to fit on the train when it stops.

If you have 100 people waiting on the platform and the train can only hold 50 people, the fare is too low. If the train has room for 100 people, but only 50 are waiting on the platform, the fare is too high.

But there has to be returns. Asking someone to metaphorically dig a hole in your backyard for no reason other than to watch them sweat is gravely detrimental to society.