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.
I think 10% more or less Avengers 1 is still flat if ignoring inflation.
There seems to be a barrier between 1.2bi and 1.5bi. I can't see a movie that is not a light in a bottle success with people all over the world crossing Jurassic World numbers.
Quality is not much of a factor when you see Iron Man 3 and AOU performance.
So far I haven't see anything new or exciting about IW. Of course Disney marketing machine can grind everything into an must see event.
But who am I to judge? Im getting old and I have realized I'm not the target audience any more.