Not necessarily true. "Shepherd satellites" can maintain rings for many 100s of millions of years. NASA was more delighted than "shocked" because the rings hadn't been observed and nobody anticipated them. In fact, their presence cleared up the long standing puzzle over why only Saturn had rings.
It (the Kuiper Belt) doesn't exist. Neither does the Oort Cloud. They were both theorized to try to explain where comets come from, but there's no evidence they exist.
There's plenty of evidence for the Kuiper Belt -- Kuiper hypothesized it on the basis of patterns in hundreds of comet orbits. Over the past 10 years or so dozens if not hundreds of Kuiper objects have been observed and catalogued. The Oort Cloud is another matter, but it's a good hypothesis in that it explains several observations without contradicting physical law.