Its pretty proven that niches can gain substantial visibility ie, a twitter, and the hope is that alone can break off and big a big cash cow, and you better do it while the trend is hot.
Most really good ideas are based around perfect timing around the convergence of technologies and mindsets. Your right about the endless signup fatigue people experience but openid tackled that and still hasnt caught fire probably because its trying to change the behavior of the user, and it hasnt integrated itself into every angle where a user could use it.
Lately I've been seeing web2.0 startups like art. They are expensive art. People are addicted to going to gallery hops to see all the new artists and their related works. Its a big show off fest. With the barriers to create and maintain so low, these pieces of art could be around for a long time, even if they make nada, they can still be there, we see that now. Eventually they will fade or other artists will snatch them up either to destroy that piece of art or to play off of it.