I'm not sure this is proven yet. Certainly in the past, I'd have agreed, but with communication via the Internet making realtime decision-making between all members of a democracy possible, I think direct democracy is theoretically possible at least.
If not absolutely direct, then at least having the ability to instantly revoke a mandate from someone you have entrusted could go a low way to limiting the power of an oligarchy, as could entrusting different individuals and groups with a single person's vote, depending on the issue. For example, maybe I'd trust a particular economist to vote on my behalf for specific economic proposals, but I'd want Greenpeace to vote on my behalf for anything to do with, say, baby seal clubbing.
So long as the individuals can instantly take back the voting power they have entrusted to someone else, a reasonable semblance of direct democracy could work.
In practice, of course, it would be a hell of a mess to get everything functional, I just don't buy the notion that it's impossible in a world where everyone can communicate with everyone else essentially for free.