Time is an important commodity in the transit market. 4 days also means requiring more planes to service the same amount of people along a route per day. Fuel isn't the only factor airlines need to optimize for, and timely travel is an essential point of value for both the airline and its customers.Küchemann wrote an article in the mid-fifties in which he showed that in the mid nineteenth century people in England mostly married people who were born within two hours train travel and that later that century it became the lesser distance of two hours bike travel. That's why he wanted to develop aircraft that would travel in two hours from England to Australia. And what about being at the airport three hours before departure?
I think we should maintain the current cruising speed for short and mid range aircraft. In current large aircraft many passengers are already unable to look out and I see no objection to passengers looking at a video or using the built in screen as their computer monitor and so BWB aircraft of one kind or another can well be built without windows. But only if the tickets are significantly cheaper.
For the long distances flying will remain boring unless passengers are able to walk around and possibly sleep confortably. The way to reduce fuel consumption and avoid CO2 production is then the a liquid hydrogen fuelled airship. Flying at 200 km/h a flight from England to Australia will take not two hours but four days. But the fuel fraction for such a flight would be less than 3 %!