The slumbercoach discussion comes up every once in a while. I agree it'd be great to have that option again, but it still can't really take the place of coach. Slumbercoaches were compartments for a single person, so if you're traveling with someone else, you'd still have to spring for a roomette - and coach passengers have that option now. The reason most coach passengers don't buy roomettes now is that they can't afford it, or don't want to pay for it. Many of them are short-haul passengers who have no need for a bed, or privacy.
I'm not sure how many people really travel solo in coach who would *also* be willing to pay a premium of any kind for a private compartment. I'm sure the answer is "more than zero" but I'm not sure it's enough to fully ditch coach and make it worth it for Amtrak to bring back the slumbercoach. All-sleeping car trains of the past assumed mostly origin/destination ridership (the 20th Century Limited only made five intermediate stops, for example, and the first four were pickup only), which most people don't do anymore.
Then again, Amtrak *is* chronically short on sleeping cars in general - they know there's more demand than supply. (That's why they're building more of them, but not more coaches yet.) There probably is about a sleeping car's worth of passengers on a lot of trains who just couldn't get accommodations and are slumming it in coach. But that still leaves 2-3 coaches full of passengers who are there by choice.
All that said, if I was still single, I'd be slumbercoaching all over the place if I still could. I used to travel alone on Amtrak when I was a kid all the time and I went slumbercoach a bunch of times and it was great. I have real nostalgia for those cars, and I actually feel kind of privileged to have been able to ride them; there never were that many and not many people these days have had the experience. I do remember the premium over regular coach going from NY to Chicago was only something like $23 at the time.