Yes, @poet has explained this :-) It's a blockchain! There is NO WAY we will lose our posts. Basically our posts are backed up on many many machines.
If you wanted to understand the 'risks' it's therefore not in losing, but in never being able to lose your posts. Everything you write and say on Steemit.com cannot be altered after 7 days - if you ever wanted to delete something it would not be possible.
We've arrived in a new era ;-)
RE: Do you have a back up of all your Steemit posts? If not, why not?