Linked below is an article where Airbus discusses the future of the A380 as a high-density carrier. John Leahy sees the A380 serving many high density routes. Airbus may yet break even on the A380.

VLA and how they might compete with development of HSR

This article has got the attention of a blog discussing HSR in comment No. 35 of this posting.

How long have high-desnsity 747's been working in Japan? Lacking significant cargo, and a generally short flight, many types od modifications can be made to existing (2 engine) aircraft allowing them to do the job at a much more effective and economical state than a 4 engine plane. If, by 2028, we are still designing/deploying new planes with 4 engines, then current propulsion tecnology seems to be the limit for the next 30 or so years, and no improvements are being executed or even sought. Not a likely scenario.