Steemit Inc is just one of players in this ecosystem and even if they are the one who wrote the codes for the blockchain, the Witnesses are the one who will approve and implement it.
We all know that Steemit Inc owns around a quarter of the total Steem in the network, there are still other investors that hold a significant amount of Steem in this blockchain that will keep things running.
The good thing about the Steem blockchain is that it is an "ecosystem", owned by everyone who hold Steem or SP, and it isn't a " monopoly" which is controlled by a single entity.
Though @Ned is the brain behind steemit, steemit wouldn't have been where it is today without the investors and bloggers that believe in the project and are keeping the platform lively and running. We know that Ned and steemit Inc are still playing great roles in managing steemit, but the role of the people (communities) in steemit can't be overlooked.
For now, I'll say steemit need Ned and steem Inc to keep running. But a time will come when it will grow very big and it will no longer need Ned to be there any more in order for it to survive. As for steem inc, it will always be there because it's the company managing and maintaining steemit. Steemit inc srevival is hinged on steemit survival because without steemit, there'll be no steem inc
Steemit website maybe will not exist if Ned decide to make it down. But Steem blockchain network will be there, if at least one full node is still alive.
If Steemit Inc goes down, Steemit will cease to exist, but the blockchain will still exist as long as there are witness to run nodes and keep the blockchain alive.
How valuable Steem will be, is not completely encumbrant on whether or not Steem Inc survives. There are many factors that go into the market as a whole, but Steem will always maintain a speculative value as long as the currency remains.