The thing is, they already have great codeshares with carriers like NWA who can offer the flowthrough to international destinations for Alaska FF
's, while securing Alaska's position on the west coast through their (NWA et al) own codeshare on Alaska.
If Alaska had already jumped up to the 757, which some of us thought might have happened a long time ago, then I could see the 787 perhaps making a showing. Alaska, however, has shown no interest in growing beyond the 737NG family, and apparently hasn't been knocking the door down trying to get the 737-900X or anything larger.
787 would look awesome, though!