'01 Prius 139,000 miles Running Well, Now What?

Transfer case? You got one of the rare 4WD Gen 1s?

I don't think I replaced mine quite that early, but yes, it is a thing to think about. Maybe it would be worth buying one to have around. You wouldn't necessarily have to work the replacement into your schedule right away, but you could do it if a nice warm I'd-like-to-work-on-the-car day comes up, or if the old one ever stops pumping.

As long as you are checking regularly, that should be fine. I do a quick brake inspection every time I rotate tires. That's not just measuring pad and rotor thickness, but also checking that the slides slide easily, the rubber boots aren't cracked, and the pads move easily in the springy clips (officially the "fitting kit") holding them. I rarely spend any more time on it than that. If the slides do slide easily with a good greased feel, I don't bother taking them out or fussing with the grease. (They will move stiffly at first, that's normal; a few in-n-out motions will wake the grease back up.) If I find anything at all that could stand to be replaced, as often as not it's just the fitting kit (about a $15 investment, does two wheels).

Keep up with that kind of basic inspection and the pads will last a long, long, long time. (Even better, if you're measuring them each time, you can plot those points, lay a ruler on them, follow it to the 1 mm mark, and see when you're going to replace them.)

But if you don't do those basic inspections regularly, there are any number of small problems that can crop up, not be caught, and result in eating up some pads or rotors way earlier than you were expecting.

They're kind of a wild card. They'll go when they go. By the time my Gen 1 hit 15 years and 23x,xxx miles, I had replaced two of them, a rear and a front. The other two were still fine. The two that wore out, wore out a few years and several tens of thousands of miles apart. I don't see much point in replacing them except as needed. When you need one you can hear it, and it doesn't stop you from driving the car (as long as you don't try to set a world bad-bearing distance record).

Mine needed front stabilizer links at one point. I caught that while replacing that front bearing.

Sounds like you have no codes at the moment. Ever done a compression or leakdown test? (The leakdown method is a little easier in a Prius, as you don't need to find a way to make the engine crank, plus it's quieter and you can hear things that help interpret the findings.)

Super easy to do if you put it on the same schedule as changing the spark plugs. Like the brake inspection, once tells you only so much, but if you make it regular you can compare and get an idea how things are holding up.

Not much to stop it, unless ... Indianapolis? ... keep an eye on the rust.