That's an interesting argument, but I'd argue that the Mariners will fight it tooth and nail because the size of the audience dictates advertising prices. Dad (who worked in baseball for years) said he doubt it would never happen due to market size in portland and the Mariners not wanting any competition in the area. This is the biggest reason why Vancouver will never get a team.
The Mariners can fight, but much like Angelos did they only get 1 out of 30 votes when it comes down to it. And while I'm sure MLB would like this to be unanimous, I think they only need a minimum of 75% of franchises (23) to vote in favor. And given that John Stanton has only owned them for a little over a year, he may not have built enough alliances yet to form a voting bloc.
As for Portland being too small, it is currently the 4th largest metro area without an MLB team in its market, behind Charlotte, Orlando and San Antonio. However, the difference between the largest of those (Charlotte) and Portland is less than 50,000 people. Not really that big a difference.