You walk. Riding a bus means you have to wait for one first. A 5 minute walk isn't worth the wait unless there are extra conditions (no walkways, ice on the walkways, heat wave...).
Above a certain frequency buses become extremely unreliable and dwell times start impacting travel time because bus stops only have so much room. With trains you have more room to work with, but once you run out the marginal cost of building new lines runs in the hundreds of millions, if not billions of dollars.
When services are made free, people have to wait longer, paying with their time instead of with money.
People who don't want to wait and are not given the choice of paying for faster service end up having to buy cars instead, making traffic worse for everyone.
I don't think you can write off the decrease in foot traffic free public transit would cause.