What is the average mileage expectancy of a Subaru?
You have to define the owner before you can make an "average" call.
An "average" car owner buys a new car with a warranty, and a monthly payment. They have "average" jobs, mostly being desk operating do nothings, hopefully making above average money to afford a Subaru (because we have to admit a new Subaru is a bit more expensive than a Suzuki, or a Honda Civic). These types of folks trade their cars every 3-5 years, so the "mileage expectancy" does not come into play.
If you do your own work (tinkering?), and have (or make?) a habit of catching a "problem" before it becomes a disaster, plan on owning a car for 10+ years, and have good mechanical "karma" (for the want of a better word), then the limiting factor becomes the effects of rust and corrosion.
(there is not a Subaru in attendance here with less than 300,000 miles, and one with 400,000 due to retire because of rust)