Think of transcoding as making a color copy of a black & white xerox.
You can push the quality as high as possible on the color copy, to minimize losses, which drives up the price tremendously, but it will always be noticeably worse than the "original" black and white xerox, and no matter what you do, you'll never be able to overcome the loss of information incurred by the first copying process.
In addition, the color copier is probably good at certain things that the b&w copier sucks at, and vice versa - but in the end, you'll always have the WORST of BOTH processes, compounded.