I would agree with your mechanic. These things make sense, even though the water pump is not in the Toyota schedule.
I worked in the Middle East and there the contractors replaced ALL DRIVE BELTS on the outside of the engine every 2 years due to the effect of the heat.
Your “antifreeze” is really engine coolant, and it needs periodic replacing no matter where you are. It contains rust inhibitors and water pump lubricant, both of which get gradually used up!