I wonder at what rate they'll need to increase the pricing in order to maintain it. Ironically improved traffic may make driving more desirable.
They will have to increase the price eventually as demand for transport overall rises. The point of the pricing is to deter driving enough that the street network operates within its capacity limits; if driving becomes more desirable than status quo ante, they aren't charging enough and will have to raise prices to keep demand manageable.
Think of it this way: either way, traffic will reach some equilibrium. The question is, what is the limiting factor? If using the road is free, then the limiting factor is traffic congestion. If you widen some congested streets, the limiting factor is *still* congestion, so eventually a new equilibrium is found which features traffic jams with even more cars.
The only way to build your way out of this limit, is to add *so* much capacity to the street network that it far outstrips any conceivable demand. This works in a number of US cities, but they're small and have an extensive grid-based street network with few natural barriers like rivers. There is simply no way to retrofit such a street architecture into a city of 8.5 million people where land costs six million dollars an acre.
So imposing use fees is really is the only way to alleviate traffic for a major city like New York or London. This raises economic fairness issues, for sure, but if you want fairness, you can have everyone suffer, or you can provide everyone with better transportation alternatives, but not necessarily the same ones. Yes, the wealthy will be subsidizing the poor, but they themselves will also get rewards well worth the price.