Transit use is good. Unless transit is at capacity.
In theory, there is no capacity for transit - just keep shoving people in the trains. Just keep boarding buses. But in reality, people don't want to stand for 30 - 75 minutes on NJ Transit. People don't want to be sitting with other people's asses in their face. Sure, it occurs often on a NYC Subway, but the passengers on NJ Transit's rail lines expect something different...especially when they're paying upwards of $16.75/direction for a ride (Trenton - NYC Penn Station. A monthly pass brings the cost down to about $12/direction).
There's also parking limits - some lots at the smaller stations are already sold out of spaces and/or permits. There's probably room currently at the larger stations, such as Trenton, Hamilton & Metropark. Most bus stops don't have any parking, so people will need to walk to get to them.
And when the trains fill, NJ Transit can't simply add more trains to the line. First, they're not all NJ Transit's lines. The main track going into NYC is Amtrak's and they get first priority. There's only so many trains that can travel on the line, and only so many terminals in Penn Station to use.
Buses also have a finite limit. The buses that enter NYC have to stop somewhere to discharge and load passengers. The Port Authority Bus Terminal has had issues with capacity problems in the past, and the terminal is going to be replaced soon which will probably temporarily reduce capacity. The XBL heading into NYC has been bumper-to-bumper for decades. Current bus usage is probably still down due to the Corona, but it's going to go up again as workers continue to return to the office.
Before NJ Transit increased capacity, Amtrak allowed NJ Transit to utilize their trains, especially their 'Clockers' which departed about every hour (these trains don't exist anymore). Amtrak may be willing to assist in that respect again, but there's no guarantee.
So, if the congestion pricing keeps people from driving, and they turn to mass transit instead, how much can the existing mass transit systems absorb? Everyone cries "just use mass transit", but if there's no parking spaces at the stations, and people are standing in the aisles for an hour or more, that's not exactly a winning solution to the problem.