And he explained why that's a terrible idea. The Nassau Street line won't see high ridership no matter what you put on it - because it's surrounded by other lines. By comparison, a Water Street subway would draw new riders. Please read the EIS, they got this one right.
As has been covered before, repeatedly, it would also be difficult to connect the Nassau St line to the Second Avenue Subway. We live in a three dimensional world, and the relative heights of the two lines won't match up, to say nothing of the sharp curves that would be needed.