Assuming they move to other modes of transport (public...transport...even?), that revenue could very will just become fare revenue. That's the brilliance of it – you have an alternative in place so that if people are so discouraged from driving, they'd take a train or a bus (which we have lots of). These people likely still need to travel into the city, and if even a small fraction of the drivers decide to simply change their lifestyle, a vast majority still either will pony up the fee, or will switch to another mode of transit.