The problem with CF isn't that our lungs fill up with mucous so much as the mucus is thick and sticky and it is difficult to remove that mucus from our lungs as it is produced. Being that it is difficult to get out when bacteria and such get into our airways and into our mucus they tend to stay there and they multiply and start to do damage to our airways. The damage that is caused from repeated bacterial infections because we are unable to clear all of the sticky mucus is what causes problems with CF - we don't get "full" of it so to speak.
As for damaging the organs/cells that produce the mucus you would do more damage by doing that I think. Our bodies produce mucus - even healthy people's bodies produce mucus - to help lubricate organs and help to remove irritants in our lungs and other areas such as sinuses, digestive and reproductive systems. Unfortunately the lubrication is something we need - ours just happens to be thicker and stickier.
I hope that makes sense. If I am wrong I am sure someone will clarify - as I don't know for certain on some of what I said as I do not have a medical degre. It is just what makes sense to me as far as our body needing lubrication in some places.
Take Care,
Lindsey