I've only been on a few long-distance trips, totaling maybe 20 or so meals in the dining car. Sometimes by myself, but usually with my wife (especially for lunch and dinner). As much as people claim to like community seating, and meeting new people in the diner, the simple fact is that the overwhelming majority of people don't want to eat with strangers. This can be demonstrated by the fact that of the 579,012 restaurants in the United States, the percentage of restaurants that offer community seating is in the neighborhood of zero point zero zero percent.
Also, as the OP said, many people would like sandwiches or salads, or other, lighter fare for meals.
So, I have a brilliant idea: food service "To Go."
Leave the community seating & old-style food for those who like it. However, Amtrak could also offer bagged meals that passengers could take back to their seats. SCAs, instead of taking seating times, could take To Go order cards. You'd then go to the dining car at your assigned time, pick up your dinner, & return to your sleeper, coach car, Sightseer, or whatever.
Some meals, like steaks, would not do well bagged, but many meals (burgers, sandwiches, etc.) would do fine.
I think it would save Amtrak on costs too. Fewer servers would be needed, and if it gained traction you'd need less seating (dining car tables could be turned into seats & sold as revenue).
If given the option of eating a diner-style meal community seating, or a nice bagged Chipotle/Five Guys / Panera Bread style meal in the Sightseer lounge or in my sleeper, I'd take the To Go meal almost every time.
