To be long term sustainable Steem Power has to be seen as more than a financial vehicle. There are perks to holding Steem Power within the system, and indeed you must own some to use the system at all.
In a zero-growth scenario the 5% loss per year could be seen as the upkeep cost of a collectively owned asset. And it may be that the benefits of Steem Power are used elsewhere to still make a profit for holders, although that's highly speculative.
In that scenario, those who are just motivated by financial growth of the asset will certainly leave, but it may be able to eventually reach equilibrium in a no or low growth paradigm.
RE: How can we make Steem sustainable in the long run?