As somebody who is working on exactly this problem, I’d say the problem can be solved technically if one can get a high density of passengers, while providing privacy and comfort.

Most of the sleeper startups basically just work with renderings, we work with iterating on full sized mock ups. We did ergonomics/market testing with hundreds of test users. We have evidence that with the right cabin technology, you can be profitable, even produce a margin, and significantly disrupt air travel.