I think there are two things at play here.
1. How do you define rebuild? Are the Houston Astros rebuilding? Is Miami rebuilding? I don't buy into the thought that you get rid of every MLB worth a damn and trade for prospects. Trade some, sure, especially when you are Milwaukee and won't be able to resign guys like Fielder.
2. What I've seen out of Milwaukee the last several years I would think meets your objectives exactly. The farm system was in decent shape for a bit. You had Lawrie, Odorizzi, Jeffress, LaPorta, Brantley - guys that looked like they had potential. in 2008, when the opportunity to "go for it", as it were presented itself, they made a move with those assets. It paid off extremely well as the Brewers made the playoffs for the first time in 26 years, and provided one of the most exciting weekends of sports I've seen with that last series vs the Cubs. Again in 2011, they traded some farm guys to make a run. Then just like you are advocating, once it was obvious they weren't competing, they turn Greinke into Segura.
I don't think I'm philosophically that far off from you, as you may think. I honestly just don't see how you don't see the Brewers doing exactly what you are advocating. They made a very solid move to try to compete. If the Brewers regress back to the mean, their SP from last year should be better, now add Garza, and this is a solid rotation. And I am confident that should the first half look bad, they would move Gallardo and Lohse for sure. And restock the farm system, just like they did with Greinke.
The biggest problem right now, is the Brewers scouting department is not good. They miss consistently on pick (see above post on Fernandez vs Jungmann), they draft a kid they didn't know had diabetes. They do virtually nothing to scout Latin America. So, until they start drafting better, they have to make do with what they have.
That said, I'd rather compete with the roster as is through September and beyond vs hoping they suck and trade guys to restock the system.
I've said it multiple times - but go look at the guys the Brewers gave up for Sabathia, Marcum, and Greinke. Those teams were trying to restock as you describe, and really all those teams have to show for it are a replaceable CF and a weak hitting 2B with roid rage.