I'll debate this while trying to be fair and impartial.
The simple fact is if Shanks defeated/killed Teach, everything he did in the story would be utterly worthless. What does it do for Shanks? We already know he is super powerful, and even if he and Luffy ever fought (I doubt it), him defeating Teach does nothing to make it more eventful.
Whereas Teach killing (or crippling him even more) does the opposite. Luffy hates him for sending Ace to his death, but now it's deadly personal. Also, defeating someone we know who is very strong gives him and his crew a show of what is to come for the audience later.