Problem is there doesn't appear to be a market for fuel efficiency.
Modern engines are built for performance, not economy, you don't get both, particularly with the Japanese who get a lot of that performance thro increased revs.
That Matchless might have had economy, but try revving it to 12,000 rpm
A good start might be for manufacturers to have to publish fuel figures, the same as car manufacturers. Until then, bikings green credentials are dubious, to say the least.