Portland, Seattle, and Vancouver have the types of demographics that make soccer far more viable than most cities. I can't speak for the Whitecaps much, but I have little doubt that the Sounders and Timbers will grow before they shrink in popularity. What you're seeing now is the die-hards that have been largely following the team since before the MLS. Expansion teams with preceding followings and sustainable demographics are going to be what the MLS needs if it's going to continue to grow from complete joke to quality league (it'll never be able to top the best European leagues, but it can be a top 10 worldwide league).
Even as a casual soccer fan from Pittsburgh I can definitely say that what the Sounders have going for them is definitely not 'new team smell'. Plop an MLS team in Pittsburgh and they'd probably attract 15,000 for their first game and then 5,000 for the rest of the season. The MLS is at it's best in the medium-sized markets and the west, especially along the Pacific Coast.
That doesn't mean that the Sounders or Timbers are about to become the city's #1 team overnight...but they're going to have better support than most of the older teams in the league out of the gate and will only grow.
For the record...I'm a Sounders fan.