I agree with you that the water pump should last another 90,000 miles until the next timing belt.
But, changing out the water pump adds almost nothing to the labor charge when they’re changing out your timing belt. The timing belt drives the water pump. So instead of risking that the water pump lasts another 90,000, most people just decide to pay for the extra part now. They don’t want to risk having the water pump last only another 50,000 miles, then they have to pay for all that labor again.