This is a great way to destroy public transit!

The plan is to let anyone on the subway, with no fares. It turns out that the violent, unstable people who make public spaces unwelcoming are disproportionately represented among those who cannot pay $2.50 for a fare. So they scare off regular transit riders who mode shift to something else. It's vastly more expensive and difficult to react to violence on the subway than to keep violent people out with a small fare, so it's financially and logistically impossible to actually keep it safe. Then your transit ridership tanks and people who hate funding transit can declare the whole thing a giant waste of money and any new funding dies politically because now there's no public ridership constituency left.

Nobody cares about the revenue gained from $2.50 fares. What matters is the tiny amount of resistance that offers to the small number of people who are largely responsible for making public transit unwelcoming.