This is an old trick. Means testing. More costly bureaucracy, just to keep an unsustainable autosprawl system going. By making all buses fare-free, a city will save much more in reduced congestion, less parking subsidy, less pollution, and better health than it would "lose" in fare revenue.
The Boston Globe: "As the Massachusetts Bay Transportation Authority considers fare increases, advocates for low-income residents are encouraging the agency to implement something new in the state: discounts for those customers who have the hardest time paying."