Your argument is like saying the more computers you copy a file to the worse the quality gets!
The data from the sensor does go through on-chip processing, e.g. to reduce fixed pattern noise. This is one of the techniques that makes the Canon images so much cleaner than competitors. The data from the sensor can get reformatted without losing any data integrity - it's just a format.
You can add metadata without losing data integrity. It happens all the time in other areas of signal processing, data capture, etc. My company does a lot of this stuff all the time.
Malcolm
You miss the point. Regardless of what metadata's added to the raw sensor data, a RAW file contains the individual sensor cell readings, not a "true" colour. So a RAW file is dependent on the sensor layout and the camera's metadata. But the image proper isn't affected by any metadata. That may be used in post-processing, or in in-camera processing of the JPEG, but it has no effect on the raw sensor data. Long Exposure Noise Reduction may be applied to the raw signal, but that's applied on a pixel-by-pixel basis too. Again, it doesn't affect the file format. Sharpening, colour boosting, contrast changes and the like are only applied in post-processing, whether to the data stream in-camera when you're shooting JPEG or in the computer later. But the thing is, since a RAW file contains both direct sensor signal capture and camera-specific metadata, the structure and content of which changes from model to model, a "standard" RAW will lose metadata from a new camera.

