Not really.
TDK had good legs, not 3.5x but 3.36x. I think you mixed it up with SM1 which had one of the best (third) legs for a comic adaptation so far (in the last 20+ years). Based on the drops so far, Avengers is looking at a run closer to Thor's rather than TDK's, for obvious reasons. That still gives it 570M+ which is not a small number by any means. That would mean that Avengers will have the weakest legs in the top 5 and top 10. What he probably meant was that people are now reeling in their estimates, when original some had thought this would outgross Avatar (lol), Titanic (w re-release), the chance of that is now slim to none.
As for THG, it still is an impressive run. 2.65x multiplier is nothing to sneeze at when you consider the movies in its genre hover around the 2x mark. Not only the legs but the opening weekend also had amazing numbers, especially for the first book of the series.
Thor is a bad template for the Avengers. Thor held up well in its second weekend, but was killed in week 3 by Pirates 4 which played to the exact same demographic. Pirates 4 also took most of Thor's 3D venues, opening in 4100+ theatres. Then the week after both the Hang-Over 2 and Kung Fu Panda 2 launched, taking even more screens from Thor. On top of that Bridesmaids and Fast 5 were still going strong.
What Avengers has going for it:
1) No major 3D competition until Memorial Day weekend. Avengers will keep all those 3D screens this weekend.
2) None of the holdovers besides the Avengers are making any money. Dark Shadows will be lucky if it makes 15M this weekend. All other holdovers will be under 5M.
3) All of the new releases added together will make less than Pirates 4 did.
4) It's still making big money, and is about the only family option currently (unless you want to count battleship).
Avengers will continue to hold onto a large proportion of screens and showtimes than Thor because it is making theatres a lot of cash, and there are no better options yet. I expect the Avengers to drop better than Thor this week and in its fourth weekend.
There is still no reason to doubt that the Avengers will continue to keep pace with the Dark Knight percentage wise for the next two weeks (especially with the holiday weekend in its fourth frame). That will be enough to push it to $600M, even if its has less than stellar holds in June. At this point I agree that a 3.36x multiplier for the Avengers is far fetched. That would give it a final tally of $695M. But I also think your $570M is lowballing things. The movie is going to pass $500M by the end of Memorial Day weekend.