Replacing the water pump on a Toyota Sienna before it fails?

Did you ask how much extra the water pump will cost?
Sounds like you went to a dealership.
There’s no need to take an out-of-warranty car to a dealer for this kind of work.
You may find that a good independent non-chain shop can do the 90K service with a water pump for less than the dealer price without the pump.
You can look for a good mechanic here: