Copernicus did not predict new effects. Even his calculations were less accurate than that of astronomers working on the Ptolemy model.  But he explained things better. Rather than keeping on adding new rings he was able to make simple sense of the solar system by postulating that the earth goes around the sun.No new rings were needed.

Newton did not predict new effects. He simply explained the same force of gravity we see here on Earth is also in the heavens. It made more sense to explain the Kepler laws rather that leaving them unexplained. But there were no new predictions.

String theory explains things that do not and can not work in quantum gravity. That it has not be developed enough to know what new predictions at present day scales that can be tested  is not a lack. And it is testable.  See this: https://physicsworld.com/a/quantum-gravity-could-soon-be-tested-using-ultracold-atoms/ So string theory or other approaches to gravity can be tested.