During the time we had all through cars/trains on the Springfield trains, we had some pretty good ridership even with high Amtrak fares. But with the loss of through service, ridership will suffer especially when the connection is not guaranteed.
That whole thing with through cars to Springfield was completely retarded. It was somewhat manageable for northbound trains, but for S/B they had to have the two cars sit on the relay track for 20 min for the corridor train to arrive. The whole scheme played hell with connections and was always a hassle having to roust BOS riders from the SPG cars.
As a heavy user of that service I can tell you that the current accross-the-platform system at NHV was a welcome relief and is much more convenient than the old through cars.
Regarding changes to the Vermonter I can say that right now the NEC portion of the Vermonter is a snare for the unwary corridor traveler. It's short consist at popular travel times is disruptive and leads to unpleasantly crowded conditions, especially on weekends. I avoid that train like the plague and always regret choosing it on the rare occasions I do (although it has a shorter schedule time).
Replacing the Vermonter on the NEC with a full Regional would not necessarily be a bad idea, but Amtrak needs to handle the connection at NHV properly or it will be a huge bust. If they could guarantee connection times at NHV of under 15 minutes I wold day more power to them, but I know they can't. There will probably be at least 30 min of padding there and transiting between the NHV platform and station to wait is rather irritating (although the layover would allow for a nice lunch stop).
The other issue is capacity. The Vermonter is often sold out during popular Vermont tourist seasons, which can actually be most of the year as Vermont has stuff to offer in Fall, Winter and Summer. Are we going to see 6-car DMU sets? If Amtrak leveraged this with an increase in service, perhaps 2 round trips a day, I would definitely take the additional trio over a through train.