Well steem is a blockchain, and as such has many more backups than a website. It isn't hosted on one machine, it's hosted on every machine running a node.
So unless one early morning, suddenly all nodes vanish simultaneously, or somehow get corrupted, there is no way for your work to be lost.
That said, I agree you should probably back up your posts, even just for the fact that going back on steem blockchain to view all your past posts is a pain in the ass.
Thank you, that goes to show how little I know about blockchain. I do like having all my posts somewhere handy so I can refer back to them, so I will still keep them in Word format, but reassuring to know it won't just disappear overnight.