I simply do not see #4 happening. Hell, at this rate I'm willing to bet that StackExchange, itself a rather niche site (or network of sites) has higher activity than Quora.

I don't know a single person outside the (somewhat incestuous) startup industry that uses Quora. In fact, I do not know a single person outside the tech industry who knows about Quora.

Claims that Quora has achieved good traction are, to be polite, overly generous, but more frankly, bullshit.