User blog comment:AYET/Friends and enemies in the New World/@comment-10543621-20150712233616

The fact Luffy is unpredictable himself, makes guessing his reaction to the news of Shanks being targetted hard. On one hand, I expect him to nonchalantly go 'Shanks isn't that weak' and pay no attention. On the other hand, I kinda expect him to go "WHAT? NO I'M SUPPOSED TO DEFEAT SHANKS MYSELF!!!" and head off to stop the alliance.

In any case, if Shanks ends up being in real danger, Luffy will definitely intervene I expect. But what you've mentioned about them weakening Shanks for BB to take advantage is a real threat and something I'd absolutely hate, but yet has a rather strong possibility. If BB takes out Shanks as well, on top of everything he's done, you can be rest assured he's gonna be the final boss. He'll definitely take the cake as enemy number 1 for Luffy,

Still, I expect something like that to happen towards the end and not anytime soon,