LiveJournal may not have been as big as Six Apart, but one thing they were was financially secure and completely self-funded... unlike SixApart, which has yet to make a single penny's worth of profit.
And it's not just an issue of whether they make money or not. They can go under because of lawsuits. They can go under because Google drops a hundred million on Blogger developers and forces Six Apart to try to keep up. They can go under because a prominent investor gets upset. They can go under because somebody important dies or decides they want out. They could go under for a million reasons.
Anyone want to take a stab at what percent of dotcoms go bust? Anyone?!