Having though about this some more, I'm not sure it's a good idea or that it will help with anything.
1. More roomy seats at the economy price point is the one competitive advantage Amtrak has over every other form of transportation, and I include the auto in that. If you don't believe me, try taking a 4 hour ride in the backseat of your car. Even the front seat, as I found on a recent road trip, where i spent 2 days in agony in the passenger seat of the Kia Soul driving up I-95.
2. Will this really make more money for Amtrak? My experience as a regular rider on the Northeast corridor is that that I've never been shut out by a sold out train. Thus, there are still more "deluxe" seats to be sold at the "deluxe" coach fares. They would need to have some sort of vast untapped market of potential riders who wouldn't mind the more cramped seating. OK, in the Northeast, they could compete with Megabus, the Bolt Bus, the Chinatown buses, Vamoose, etc., but they'd have to charge those kinds of fares. (I just checked Bolt bus fares - $35 BAL-NYC for today (though most buses sold out), ~$15 advance purchase. Normal Northeast Regional fares are ~50 for Saver, usually have to pay $80-$130.) If you sell seats with bus-style seat pitch, you may get lots of new riders if you can keep the fares low enough, but how much revenue will you lose from existing riders, who are now paying the big bucks, but might be tempted to use this new economy class and save some big bucks?
3. The Palmetto and Pennsylvanian already run Amfleet I cars in addition to Amfleet 2 cars. Perhaps they could do a pilot on those trains and offer an economy class riding in the Amfleet I cars and see if there's a market for this on non-corridor trains.
4. Seating on the Capitol corridor in California is already pretty tight. The Pacific Surfliners are more like Amfleet Is. I don't now how it is on the San Joaquins.
5. Amfleet I coaches do have a bit more seat pitch than airliners, but not a whole lot more. Their real advantage is the seat width. I can't see squeezing 5 -across seating in an Amfleet, let alone 6-across seating. And I think that 5 or 6 across seating would not attract even the bus riders, So I'm not sure how they would configure this economy class seating.