I would say yes it could be done at 18K or even much farther, but if you had a clutch cylinder seal failure at 15,000 miles, and you did not change the fluid, you would be stuck. For machines out of warranty, I do things at the intervals that experience and common sense dictate, but for machines still on warranty, what the manufacturer says must prevail. My personal opinion is that BRP is overly cautious, but then again so are BMW and some others. I don't wish the possibilty of voiding my warranty by neglecting a specified fluid change, whether I personally feel it is necessary or not. These things cost too much too be considered throw-aways.
-Scotty