Steem can exist without a blockchain. I mean, facebook can very well create rewards in their system and even go as far as back it by the shares of facebook Inc (you like, you get some shares). You really don't need a blockchain for that.

This raise the question, does Steem need a blockchain?
I value a lot the censorship resistance and I think we will see a newspaper on steem at some point. Does anyone else care about that? I think Nope.
Decentralization is another issue. I view the witness production more decentralize than Bitcoin/Ethereum with less than 10 pools that decide everything. As for the access to the blockchain, that is where I am uncomfortable. We are building a system where the main entry point will be steemit(website, api, ...). It bothers me but it doesn't seem to bother much people around here.
But users care about the reward system, more than anything. The reward system doesn't need a blockchain or doesn't need everything to be on the blockchain.