I don't think so you really understand what you mean by steemit and dtube.
But, If steemit is down , steem blockchain will not be affected. Steemit is just a UI and not the the whole of steem blockchain. The very core is the steem blockchain. If steemit.com is down, still you can do transfer funds using esteem app, busy.org, steemitstage and others,
The witness job is operating 24/7 for confirming every action in the blockchain,
Do you want to host a server? you can do it
Who can be a Witness (server) for Steem?
It seems that you have to vote for the witnesses. This may mean that only the top 30 witnesses are actually distributing the Steem blockchain.
Steem = Bit Torrent / Bitcoin / P2P / etc?
It seems that the nodes, the servers, of the Steem blockchain, is not completely like Bit Torrent or Bitcoin which seems to allow for anybody to download the app, the program, to host the nodes, servers, the pieces of the pies, of the networks. If Steem is not already doing that, they should be.