Someone's got to fund the steemit web servers and pay to keep the witness nodes running. At some point, it's not worthwhile to pay for that infrastructure. I'm the first to admit that I have no idea how close we are to that point, but it's out there somewhere. Yes, you can always keep a copy of the steem blockchain and resurrect it with new infrastructure, but the steemit web site is a start-up business, and start-up businesses die all the time. Residing on the blockchain doesn't turn it into a perpetual motion machine.
Here, let @dantheman tell you,
Steem Dollar Stability Enhancements
This system works well when the market cap is rising or relatively stable, but there is one edge case which could cause the system to unravel. When the market cap falls and pushes the Steem Dollar ratio to dangerously high levels (20% or more of market cap), then the network must start paying off its “debt” with “equity” and reducing the Steem Dollar supply.
According to coinmarketcap.com, the current SBD market cap is 1.5 million and the STEEM market cap is 21 million, so it's not at Dan's dangerously high 20% yet, but at the rate steem is falling, it's uncomfortably close for my tastes.