Yes, you are right when it comes to IBS and for the reason you gave. But there are other reasons that the traditional colonoscopy will decrease in usage:
1. The advent of the virtual colonoscopy which still requires preparation (laxatives). The colon is the filled with air and a cat scan or mri images the colon.
2. An Israeli company - Given Imaging (Nasdaq: GIVN) has been successfully making and selling a device/procedure called capsule endoscopy which images the small intestine. The patient swallows a small pill like camera which transmits images to a device on a belt worn on the waist. They are now making and selling a device/procedure called capsule colonoscopy which images the colon in a similar manner. Again, this requires preparation (laxatives).