That certainly seems like a possibility. The Legacy is my first Subaru and I am very impressed with it, so I can see why people might want to hang onto them. Most of the guys on my Legacy forum have owned several Subarus, with many hanging onto their older ones even after they bought a new one. Also, you have to understand that compared to Toyota, Ford, or GM Subaru is a very small company with substantially less cars built per year. The current Legacy has been out here in the US for well over a year now and I can probably count on both hands how many I've actually seen on the road since getting mine. The Outbacks, Foresters, and WRX/STi's are a bit better sellers, but you still see far less of those than say a Ford Taurus or Toyota Camry. It's also a regional thing from what I understand, supposedly Subies are crawling all over the place in Alaska and other areas with severe winter weather due to their great AWD systems and good snow and ice performance. Plus they get much better gas mileage than say a big SUV, and they're infinitely more fun to drive.
Be careful with buying a used WRX or STi though. Although they are fantastic cars they're sort of like Mustangs, sometimes hard to find ones that haven't been monkeyed with, hacked up, modified, or otherwise beat on. There's a kid not to far from me that has a brand new STi and he has more crap on that car than I can count. Gauges on the A-pillar, Sparko racing seats, aftermarket wheels, ground effects body kit, you name it. Plus I see him hammering the poor thing all over town. I'd hate to be the poor guy who bought that car from him. Also I'd love to know where a kid in college comes up with over 30 grand for a new STi, must be nice.
If you're interested in an Impreza/WRX/STi head over to NASIOC, it's pretty much the center of the universe for those folks. I think they have a classified section and some good tips on what to look for with these cars.
North American Subaru Impreza Owners Club