Public transit should be nearly free - but I do think there's one main argument in favor of keeping it from being completely free - safety. Without a token charge, and with a lack of adequate shelters, busses are one of the most reliable shelters from weather and if folks flock to it to stay cool/dry/warm then it'll make it feel less safe especially for vulnerable folks including single women and the elderly.
We absolutely need to move beyond this dumbass neoliberal idea that public transit should be self funding, though.
They've had to hire three new interns just to carry around my enormous pile of tabs.