The voting system can be sustainable provided SE is able to attract and retain knowledgeable subject matter experts. A loose working definition of such a person might be someone who is capable of hitting 20k on a medium volume site.
A voting system built around usefulness and quality of questions and answers absolutely requires such people participating, otherwise the general trend is downwards, in both quality and quantity, and SE will eventually become what it'sits founders set it up to oppose.
The voting system doesn't need to change,change; SE does.