FWIW, as far as I know the Gateway Project is designed to augment, rather than replace, the North River Tunnels. The Gateway Tunnels are needed to allow for long-overdue maintenance on the 1908 tunnels, but I'd be very surprised if they aren't returned to service after an extensive rehabilitation. Given the volume of trips between NY & NJ, there's just no way they'd let them sit idle forever.
FWIW, as far as I know the Gateway Project is designed to augment, rather than replace, the North River Tunnels. The Gateway Tunnels are needed to allow for long-overdue maintenance on the 1908 tunnels, but I'd be very surprised if they aren't returned to service after an extensive rehabilitation. Given the volume of trips between NY & NJ, there's just no way they'd let them sit idle forever.