I'm a bit late reflecting on this as requested, but better late than never.
I'm not 100 percent sure of the impacts the Sounders FC is going to have on the Mariners in terms of fan support or health of either franchise. But, if there ever was an opportunity to start a new franchise in Seattle (or reintroduce an old one) now is the time.
I've written a lot in the past how a nearby MLB team would hurt the Mariners, mostly because the Mariners t.v. contract (plus really good local ratings) make them a money-making team. If you carve off a good chunk of that t.v. market, then the Mariners make less. Two mediocre baseball teams instead of one good one. Three years later, not even one good one.
I'm not sure if the same argument applies to a soccer team vs. a baseball team though.
I'm not even sure if an MLS team in Portland would even threaten the Sounders market, but rather since soccer is relatively new to many markets, a close rival would probably do more to increase interest.
Another reason isn't necessarily a reason, but a sort of sign post. If baseball executives aren't worried about a nearby MLS franchise, why should fans be? The owners of the Oakland Athetics are so not worried about a soccer team with a congruent schedule that they bought one. The New York Mets' owners were also said to be interested in a summer soccer team, despite the crowded New York sports market.
In a historic note that doesn't really address the question, baseball's connection with soccer goes way back. In the early days of professional North American soccer (I mean the early days, not the 1960s, but the 1920s) baseball owners saw soccer as a way to extend the profitability of their stadiums. By playing soccer during the baseball off-season (when North American soccer still adhered to the world soccer calendar), they could make money year round. (see page 55)
Ever hear of Fall River FC or Bethlaham Steel? That effort did not work out.
Just a last few thoughts on schedules. Soccer in North America doesn't exactly have the same schedule as baseball right now and might not in the future in a much larger sense. While the MLS and MLB schedules overlap, MLS teams play in several other competitions. These include the US Open Cup (still summer), Superliga (summer again, my argument is losing steam) and the CONCACAF Champions League (summer to spring, certainly not baseball type schedule).
There is also talk of MLS going to a two competition league system sometime in the future. This system is popular in our hemisphere and would allow the continuation of a playoff system and allow a summer break.