Amtrak gets taxpayer support for long distance trains primarily in order to serve rural areas underserved by other modes of transportation. Most of these people are traveling in coach and aren't making long trips. There is also a public interest in providing mobility for people who can't or won't fly, often for medical reasons, and people who can't or won't drive, and getting cars off the road is in the public interest, anyway.
Premium class service, like sleepers, is a luxury add-on that makes financial sense for Amtrak, as they do yield much higher revenue per passenger mile than coach, and could thus cross-subsidize the costs of providing the public-interest coach service. An all-sleeper train, however, has a much more limited clientele than a full-service train, and it might be argued that there's really no public interest in taxpayer support for such a luxury good.
As an add-on, sleeper service is justified, because it helps the train's bottom line, but I think there's no place for an all-sleeper train in a taxpayer-supported service.