The problem is that for most people over about 21, the biggest component of "comfort" is privacy. This is why people book hotels rather than hostels, even when the hostels are stylish and luxurious. Of course, night trains can be hotels-on-wheels too. The better ones have first-class cabins with private bathrooms. But this makes no sense from an environmental point of view. At this point you might as well take the short flight.

The best couchette-style service in Europe right now is the recently introduced Nightjet mini-cabins. Capsules, basically. This is a major step forward IMO but I still don't see this tempting most normie travelers. And so expensive, too.