Rosetta/Philae returned to Earth three times for gravity boosts. Each time it was going at speeds which would guarantee its destruction if it hit the deeper parts of the atmosphere. Had this happened and Philae had carried an RTG, it would have been the end of ESA due to the public outcry, and NASA would likely be in public relations trouble too.
There are places for RTGs, but Rosetta was not it. Philae may have died prematurely, but ESA is alive to try again.