This isn't true. When John Hammond took the GM job he identified Yi and Bogut as their two most valuable players. He labeled them the building blocks of the franchise moving forward. He also praised Michael Redd to a lesser extent. His plan was to move certain players (namely Villanueva and Mo Williams) and try to make the team a playoff contender immediately. If you search through the Milwaukee Journal Sentinel archives, and listen to Hammond's interviews on Milwaukee's local sports radio stations, you'll see this.
The plan to build around Yi, Bogut, and Redd changed when Kiki came calling. When RJ became available, the Bucks wanted him, especially Skiles. They offered a variety of deals to the Nets, almost all of which included Charlie Villanueva. According to the Bucks post-game show host (on ESPN 540, Milwaukee) the Bucks offered Villanueva and a first round pick for RJ. Kiki turned down all offers from the Bucks that did not involve Yi. Eventually Hammond agreed to the Yi/Simmons deal.
Though dealing with the Chinese government can be a pain in the ass, Milwaukee had every intention of keeping Yi. Hammond and Skiles even had plans to fly to China to visit Yi, watch him train with the CNT, and watch the Olympics.
In the end, getting RJ fulfilled a lot of things the Bucks wanted this year. They had to find a SF to fill their weakest postion, they wanted a name guy who fans know to get them excited about Bucks basketball again, and they wanted to win immediately. Ultimately the Bucks decided that gaining these things was more valuable then keeping their young prospect. As Hammond said after the trade, "you have to give talent to get talent."
But make no mistake, if Hammond had his way, he would've gone into this season with a starting line-up of Bogut/Yi/Jefferson/Redd/Ridnour.