Nitpicking here, but a colonoscopy isn't a preventative procedure. Things like exercise and eating healthy are preventative. Colonscopy is a screening procedure trying to find (early) an existing cancer. I guess you could argue that it may prevent an early stage cancer from becoming a late-stage one. But that's different from preventative in the sense of preventing cancer in the first place (such as by not smoking), or using vaccinations to prevent a virus from spreading.

Also, the evidence that colonoscopies are appropriate for general screening (as opposed to screening only high-risk individuals) is not good, which is why only the U.S. and Germany currently use them in this way. In fact, it's quite likely that colonoscopies do more harm than good when used for general screening.