I disagree. I like that Oda is going from just having Luffy do everything all the time. "Character with deep roots to the villain tries to do something, then fails completely so Luffy steps in and saves the day" was getting repetitive and stale. It's good writing that Oda is going this route. It'd make no sense if Whitebeard's parallel in this generation and one of Luffy's main rivals for the title of Pirate King is someone who has to depend on Luffy. Not only that, but it's better for the person who actually has the deeper ties to the villain to land the defeating blow, or do something that is critical to his defeat. Not only that, but Law is extremely popular at the moment. Doing this would be good also because Oda would be giving the Law fans a treat. I mean, what are Luffy's fans gonna do, complain about how in all of 20 arcs, there was one where Luffy didn't do the finishing blow to the bad guy? That's some petty, fanboy shit, especially when it's compromising to Oda's writing ability.
He has faith in Luffy, remember?