Demand for Steem Power will likely be the driving force. There's two main reasons to hold Steem Power:
Financial return on investment. As long as the system is growing overall, you can expect a return, maybe an outstanding return.
Stakeholder voting. By holding Steem Power, you get voting rights within the overall Steem ecosystem. You get a say proportional to your stake in who gets rewarded and why. On SteemIt.com, you get to decide who/what rises to the top and who falls to the bottom. This is valuable potentially for any user, but perhaps marketers especially in the long run. Steem Power is also required to transact on the network at all.
In my view the long term viability will depend on whether the perks of owning Steem Power are enough for people to hold it even when the system overall has reached an equilibrium. If it is dependent on never ending fi1nancial growth, then it must fail at some point, it's just a matter of how large it gets before it can't keep growing any more.
RE: STEEM: brilliant on the supply side, what about the demand side?