I assume that the internet archives would utterly fail to back-up friends-locked posts. Also, a brief check suggests that www.archive.org, at least, has very spotty coverage for livejournals.

I would also not assume a warning. If I were the LJ powers that be, I would fear that warning people that LJ might disappear would encourage people to move off LJ onto other blogging/social networking sites, while also discouraging purchases of paid memberships. In other words, it might cause a "run on the bank" in terms of social capital, thus pushing it under. If that were my theory and I were running the rodeo, I might well keep running it without warning until I had to fold suddenly, hoping to avoid a collapse altogether.