The need for additional tunnels is necessary due to the lack of capacity. It is true that extra trains would bring extra riders. However, I don't think that the ridership numbers in this case are accurately quantifiable. New Trans-Hudson tunnels would be, in my opinion, one of those rare "If you build it, they will come" situations. Not only will ridership increase, but it is possible that the populations of the towns that already exist around stations along the lines that will gain one seat rides from this improvement could increase as well due to the newfound ease of getting to the region's economic center: New York.
And then, when talking about service on the Raritan Valley line, there are additional infrastructure capacity issues you need to worry about, such as the crossover at HUNTER, sharing track with an increasingly busy Lehigh Line, and the potential (long shot) restoration of West Trenton Line service.
Bottom line: this is a great gimmick, but it proves nothing. Focus on the current and future infrastructure needs to make it happen. The riders will come.
Disclaimer: Everything above is my opinion based on what I have read here and online. I have never ridden the Raritan Valley line.
Lines Frequented: SEPTA Trenton Line, West Trenton Line, Warminster Line; NJT Northeast Corridor Amtrak Northeast Corridor (PHL-NYP).