I think we would be wise to keep in mind why Americans are, or are not, using Amtrak.
The 4-6 berth sleepers in Europe are popular because overnight trains, despite generally operating at a slower pace than high-speed trains, actually save time for travelers and are often cheaper than a hotel room. Especially for young American tourists, you could spend half your day flying from place to place, a full day on a train, or just take an overnight and have the full day to your leisure.
Those conditions do not exist in the US. Besides the ubiquity of air travel, there are very few bona fide tourist or business destinations point-to-point on Amtrak's network that can be accomplished in an evening-to-morning train, outside of possibly the NEC. The Capitol Limited might qualify in this definition, but even that is taking up half your day upon arrival in DC, or half your day upon departure from DC.
The Lake Shore Limited corridor of Chicago-western NY is probably the best business corridor I can think of.
Given that most of Amtrak's coach passengers are using long distance trains as "corridor routes," I think the following questions are helpful:
1. Are expanded overnight offerings and capacity meant to earn new ridership?
2. Are expanded overnight offerings and capacity meant to earn marginal revenue from existing customers?
3. Would expanded sleeper offerings and capacity better convert first-time riders to regular riders?
I would think questions 2 and 3 are more relevant than #1, but I'd be interested to hear others' opinion on that.