NTD took time and I can imagine it being expensive. Parking a bunch of otherwise-unused equipment still took crews, engines, fuel and space. Then, there was the clean up and bunch of other behind the scenes operations that volunteers didn't perform. Corralling and storing the private cars also added to the expense, even though they brought in the crowds.
Additionally, I agree that although it did generate goodwill and awareness, it had grown stale. The novelty has worn off. Without the private cars or something unique, how will you hold interest? How many times can you show the same people the same equipment they usually see? Even though it isn't feasible from an operational or financial standpoint, I love to ship the Acela to somewhere outside of the NEC. I'd like to drag a Talgo set to WAS, a Cascade to PHL, ship the 8400 to Texas, the 10004 to ABQ, the California Bilevels to MIA, the 10005 to HUN, the Sounders to ALB and the 9800 to RUD . Mix it up. Let the people see something they aren't used to seeing so they know what else is out there.
Absent that, I think the concept of a city hosting an event and Amtrak showing up has merit.