Since the steemit interface for writing posts is not very good, and has dumped people's posts before, i write everything in KWrite (notepad) and save it on my computer.

Now, steemit.com can go down and has.
If it happens you can just go to busy.org or steemd.com and read your posts there.

If STEEM (the blockchain) goes down, there is nothing left. But, STEEM is a group of computers. All the witnesses are storing a copy of your blog posts. So, all of the witnesses would have to shut off their machines. Of course, a programming error could corrupt them all. Or make them all unreachable.

So, there is probably a 99.44% chance that all your data will be lost.
When you try to look at your old posts on your one hundredth birthday, you will find that you can't scroll back to the beginning of your posting.
(actual bug in steemit.com that hasn't been fixed yet)