Dealers’ (not factory) aggressive-maintenance policies are often money-driven. You should consult 1) All details of your warranty, 2) Your owners-manual, 3) Factory-Service manual. On the last item, I would go to a different dealer, ask to see the Service Manual on the Sienna without telling them the exact reason or any “leading” information as to why. This service manual may also be available online somewhere. If you are unable to locate independent information on why this replacement is necessary, and at last resort the original dealer refuses to provide such proof, they will either back-down from their position or fail to provide the independent-proof. Then you will know with certainty that this is bogus.
To the best of my knowledge and belief, unless you live in Alaska or Death Valley, as long as you maintain fluids, regular thermostat changes and engine doesnt overheat, the water pump will last well over 100Kmi and maybe twice that. You can prolong the life of the belts and prevent cracking, by applying generous amounts of Belt-Dressing (spray can) just before Winter and also just before the hottest Summer months. I would apply it twice during both periods. The engine should be running during application.
Good luck.