Why? Luffy could be doing something else at the same time.
Think carefully here.
Do you really think Oda who is a bigger Shanks fan than people like KOL and some users here, is really going to have Blackbeard defeat Shanks off screen?
A character that Oda has built up since chapter 1 of the series, do you really think Oda will have Shanks and his crew get defeated off screen without showing anything from them?
And before you say that Oda will show the fight, are you sure about that? This is Oda we are talking about, unless it's the Straw Hat fights it will always be off screen.
Akainu vs Kuzan: Off screen
Inu and Neko vs Jack: Off screen
Jack vs Inu: Most of the fight was off screen
Law vs Blackbeard: Yeah we saw a little bit of the fight but the rest were off screen
I'm sorry but there is no way that Oda is going to show a full Blackbeard vs Shanks fight on screen.