How old is your Subaru? I ask because I went through this same thing with my 2004 Forester. The salt is the real killer up here, not the cold, and once it gets into the rear wheel wells and the subframe it's pretty much over no matter how good the engine runs. What worked for me was crawling under there every fall before the first snow and spraying the frame and rocker panels with fluid film, plus hitting the car wash on warm days in the middle of winter to knock the brine off. Mine made it to 240k miles before the rear strut towers got soft enough that my mechanic told me to stop driving it. I'd say 12 to 15 years is realistic here if you stay on top of the underside, and the guy was right that it's a race, just not one you can ever win.

Friend of mine ditched hers and got a soaked floor. You get lucky?