Congress is fickle enough without rockets blowing up, even if NASA explains up front that it's going to happen. There is much which is suboptimal about NASA, not just their attitude towards perfection, which is downstream of the political reality they have to deal with. For instance, a project that could be done in one year given adaquate funding will instead be spread out over ten years or more, to spread out the costs and keep NASA's monetary requirements as smooth and predictable as possible, for the sake of Congress.