Bucks will have to give up a lot more than Yi to get Garnett. Minnesota turned down a lot of deals involving much more that. And the Bucks would have to give up enough players so that they would not be contenders.
It'd have to be something like Bogut and Yi for KG nothing else would work, really. It woud'lve been impossible to do because the Bucks would be left with a bag of carrots for depth, which is worse than what Boston has. Even after that, the Bucks still wouldn't be favorites in the East.