Back in the '90's I had to do LHR-SYD four times a year for three years.
Quite frankly, it was hell - and that was with BA in F. I well remember the feeling of complete despair as I sat down and contemplated 13 or so hours to SIN or BKK. This feeling was only made worse after the one hour stopover when you realised you still had another 9 hours to go. The one consolation was that flight-deck visits were the norm and I had the pleasure of a few arrivals sitting in the jump seat as we crossed Darling Harbour coming in to land.
Paradoxically then, doing it all in one go with today's electronic work and leisure technology (rather than luggable laptops with 10 minutes of battery power) might be more acceptable given that once you're on the way, that's it.
However, it still is a very long time to be on an aeroplane and as others have posted, the crewing logistics are difficult.
In 30 or so years, maybe it will be possible using space technology but until then, I think I'd still use the solution I found after the first year of my travelling there - two day stopovers in both directions.