I've done sleeper buses/trains in south east asia/China and my impression was you actually get used to it fairly easily. The trains were generally better simply because the beds were bigger, but I'd definitely take a "sleeper" bus over a regular sit down bus any time, lying down even if it's a bit cramped is always more comfortable than sitting up.

Have you ever taken a Greyhound? Granted I have a n=1 experience, however a Bus leaving Denver (with a final destination of Dallas) leaved much to be desired.

The route-through method means that you'll stop about once every hour for 15~20 minutes for on-off and snacks/restroom. If you're still somewhat wakeful these stops are just at the twilight phase and you don't get truly restful sleep.

Have you ridden trains in America? I've travelled almost every Amtrak route across the country. In a country that properly maintains its rail infrastructure, Cabin will definitely lose much of its competitive advantage. In America, however...

Only used trains in Europe so far hence why my impression of trains is probably a bit biased...

Not biased, you just have an impression of trains in systems that are well-funded as a public service. Come to America to experience the opposite.