To add to the reasons why for many people off-line is essential.
Many people, myself included, play Elite a bit like a rogue-like game. There is a reason these games are all single player. Dwarf-Fortress, ADOM, Nethack, etc. It is because we like to be in our world in a considered way. We like to pause and think about it. We like to know we will be coming back to it in whatever amount of time and continue.
I believe this is part of the reason Iron-Man mode was fought for so successfully in the forum and DDF - because there is a strong element of the same kind of enjoyment as rogue-likes for Elite players, and this will be why many bought it or backed it.
I know it was a large part of it for me, and was why off-line was an essential feature. Where the main game was, for me.