I think that the people of Seattle are smart enough to know that AS is a business and not their friend. Moreover, I suspect they recognize that they are a big enough, important enough, and rich enough city that they will always have decent competition.

As for connecting in a DL-dominated hub... I suspect they'd look at somewhere like Atlanta and recognize how valuable the ATL hub is to the Atlanta economy. Flying through MSP, they'd probably find comfort that, if a city like MSP can have over one hundred nonstop destinations despite playing second fiddle to Chicago in the region, that SEA can be a viable hub on a West Coast that includes LAX and SFO. Flying through DTW, they will probably be hoping that Seattle never becomes Detroit and, if they do, that some airline will still bother to keep a hub there.