Because it’s based on the assumption that congestion didn’t actually go down, see number 1 posted by op.

If you want congestion to go down, keep raising the price. It will eventually go down and revenue could go up a lot.

Or you get voted out of office and your charges reversed down to zero - or perhaps negative as the people are so mad they take it out on the transit this was supposed to fund.

Politics is tricky, don't take so much you make people affected mad enough to undo what you wanted.

Both parties like money so one party may be voted out if people are angry, but it’s unlikely to result in the charge going away.

It’s also nyc primarily in charge of it and nyc constituents probably are in favor of less congestion and more money.

Politicians like votes more than money. If this is seen as the standard change of hands that happens once in a while in a good democracy then the charge will stay because $$$. However if this is seen as a rejection of the charges they will go away to prove your vote for the new people wasn't wasted. Seen is the key here - while surveys and such influence this, there is emotion there as well. Note too that it only needs a small vocal percentage in some cases to change perception.

Big econ brained is thinking about whether the congestion pricing is approximately captures the negative externalities of traffic

First, it's not exclusively defined by revenue (which is what my first point was alluding to). Second, the underlying assumption of revenue generated is that it's going to the MTA and used to improve public transit and therefore quality of life in the city, which would be a success.