Thanks for the information.If one assumes that the usable internal floor space in a typical car with external width of 10'6" is 10' then indeed after you set aside 36" ADA required aisle width you are left with 84" which divided 4 ways is 21" which would be the width of each seat in a 2x2 configuration. OTOH, while Brightline has its 2x1 seats 23" wide, in principle in a 2x1 arrangement one could get a seat upto 28" wide without breaking the ADA36" aisle requirement.
The Siemens Venture seats are cited to have a width of 19.1 inches. Source: Faster Wi-Fi but slimmer seats: Here’s a first look on board Amtrak’s newest trains - The Points Guy
This board cites Superliner seats as "ranging from 20 to 23 inches," which probably accounts for SL1/SL2/overhauled versions.
So it would seem that maintaining current long distance seat width is possible, from these back-of-the-napkin calculations, though perhaps there would be a small sacrifice for armrests/mid-seat power outlets.