[Noriyasu]

I'll add a few things.

I've never seen WP lock up. If there is a problem with the coolant, for example a large amount of rust, the impeller may lock up due to corrosion.

I have replaced WP and timing belts on more than 10 SUBARUs which own by me, family and friends, and I think the first sign of damage is a coolant leak.

Coolant leaks from around the impeller shaft. The shaft is sealed with a rubber seal and spring, but as the seal deteriorates or the shaft wears, it seems likely that leaks will occur. The degree of deterioration is thought to largely depend on the environment in which the car is used and the condition of the coolant.

I'm not saying replacing it is mandatory, but it's a well-known fact that it's safer to replace it if you can get a compatible product.