TLJ is actually going to end up ~$730M-740M behind TFA. Which is ~$80M more than JL's take. Pretty bad when they were most likely only expecting to be down by ~$250M-$350M, which BOM called the "safe bet."
Though, I don't agree with your assessment of IW's chances at the BO. Avengers:AOU passed Iron Man 3 by ~$190M WW. Don't see why IW can't at least match that. Especially when it has pretty much everyone's fave MCU characters in it. As long as the movie isn't godawful like JL, which I don't think any MCU has been, or seem like a total clusterfuck, I think it has a good chance to have a great BO result. There's a chance it could beat the OG Avengers, but even if it doesn't, I doubt it will be much below it.
TLJ was never going to make as much as TFA. TFA was a generational event. Perfect storm. No competition. 30+ years of pent up demand for original cast. etc.
Jumaniji is the a huge difference here. TFA went up against a bad Chipmunks movie that was pretty much panned. Jumanji looks to be one of the biggest surprises ever. Probably will hit 400 million.
Combine the fact that TFA had more pent up demand and hype with the added competition of Jumanji and I think what TLJ did was respectable. Not great. But good enough.
End of 2009 Predictions (Set, January 1st 2009)
Wii- 72 million 3rd Year Peak, better slate of releases
360- 37 million Should trend down slightly after 3rd year peak
PS3- 29 million Sales should pick up next year, 3rd year peak and price cut