A few things;
1) This is not the end of cancer. Even if it is 100% effective, people will still develop cancer and then need this treatment. Cancer will always exist until treated.
2) RNA is unstable and already widely used in many treatments. It will not mutate DNA permanently, and therefore should not concern people about potential negative effects.
3) Yes there are many 'cures for cancer', some of which are fairly effective. However as we learn more about medicine, in this case micro-medicine and genetics, we can start to make better cures. Thus there are always new headlines and new developments in this feild.
4) There eventually will be specific cures for specific cancers. It is now known how lung cancer develops from a cellular mutation, and the gene's responsible for this. With more genetic engineering there will be cures that will work. Think of it this way, 20 years ago, noone thought we would be able to grow say a human ear for transplanting, but with stem cell research now this is possible. If I said 20 years ago, I would grow this ear on the back of a mouse, I would probably be ridiculed like the FEers are currently.