Here is a logic.

Replace the water pump if it has failed.
Replace the water pump if it is failing.
Replace the water pump if it is likely to fail before you next change your timing belt.

The first two are easy decisions.
The last is less easy, but I think we can reach a decision here.

The question is, are 850 water pumps high, or moderate, failure items below 140,000 miles?

If not, then I see no reason to change the pump.

If yes, then change the pump.

An example: the first generation Honda Accords with the 1751cc CVCC engines had a water pump with a singÈe row ball bearing supporting the impeller shaft. A bad design. The useful life of these pumps was 30,000 to 60,000 miles. Very few went beyond 70,000 miles. Most failed by 45,000 miles. The pumps were cheap so, whenever I was in one of these engines (doing work around the water pump) I changed the pump.

Do 850s have this same genetic weakness?