SW TLJ grossed $600 less than TFW. That is almost the whole Justice League BO.
I don´t think there is much room for Infinity War to pass Avengers 1 or Iron Man 3. It won´t have the media bliss BP is getting and will face stronger competition during the summer.
The floor for Marvel superheroes movies has gone up, but the ceiling is at the same place since Iron Man 3.
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.