Because we'll have to get young and suck eventually.
I'm not convinced that the Celtics have to go through a "young and suck" phase between the current and the next championship windows.
The only way I could see that being the case is if Rondo turns into a superstar (i.e. legitimate top 4-5 MVP candidate) when he gets back from his injury.
We aren't building a contender post-Pierce/KG without a superstar, and we aren't getting one in free agency. So it's Rondo, tank for the lottery, or acquire various young assets over the course of a few years and make some trades.
Personally, I view the latter of the three options as the most likely one. But it means we have to get young and suck for a few years (at least).