You prolly underestimate the value of the japanese market. OP is gigantic there, hardly comparable to the west, although it's also huge here.
Also, the average fan prolly doesn't care much for power levels and OP fights. The japanese and the huge casual part of the community will gobble up the movie. Especially because they love their idol shit, for whatever reason.
Also, if you remove the movie as an argument. It isn't wild to assume that Shanks simply acted like this (just appearing near Wano, but not entering it) because he might have known that the marines or other forces might show up after the news to take care of the exhausted losers... Beckman being pretty smart helps this argument.
P.s. Yeah, Oda sucks tho. Without us theorising the appearance of the red hair pirates, it looks stupid asf.