The analyst who made that prediction is with Exhibitor Relations. It's not as if he pulled those numbers out of the air. He has access to information about ticket presales, tracking data and the like upon which he based his prediction. It's less optimism on his part -- he's a professional who has no reason to talk up the movie -- than an informed estimation of the movie's prospects.
Who said that? Notfabio (a poster well-known on HSX, which is like Wall St except betting on movie box office) reported that presales at his usual multiplex, which has repeatedly proven to be a good representative for general audiences nationwide, for Avengers was "ok but not spectacular, definitely better than the non-Iron Man films."
Which isn't really that surprising, since there's less of a crazy fanbase (at least compared to Harry Potter and Twilight

) and more of a GA appeal. That points to more last-minute walk-ups than waiting months for the premiere.
Still, the audience to sustain a Harry Potter-like opening weekend is likely not there. In fact, World of KJ is even considering the possibility that Avengers won't even beat The Hunger Games' opening weekend.

Not to knock Avengers of course, but Hunger Games was a BEAST.
There's still a few weeks and things might very well blow up between now and then.
The boxoffice doesn't work like that. lol The Avengers will be it's own entity. Earn it's own money off it's own merits not necessarily piggybacking off previous efforts.
Yeah, considering people who watched Cap and IM and Thor are likely the same kind of people.

Overlapping audiences - you can't just add them all together and assume they'll be paying to see TA 4 times to make up for the fact they also saw the previous movies.
I mean, some of you surely will, but most people who see movies aren't like us.