I don't think Alaska is as strong as you think it is. There's about a six week stretch (from mid-June through July) were fares are high; the rest of the season, they're often the lowest priced 7-night cruises. Keep in mind
DCL's pricing is inclusive of taxes & fees, and the taxes & fees on those 7-night cruises are about twice that vs. the Caribbean. The number is definitely not insignificant.
That said, there have been crew rumors that Dream is headed to Alaska. Obviously we have no idea that if that happened, if the Dream would supplement or replace the Wonder. My guess is replace -- the one-way cruises between San Diego and Vancouver are historically unpopular (that's true of any cruise line between California and Seattle/Vancouver) thus many of the rooms are sold at extreme discounts to select employee groups (and obviously most Disney corporate positions are based a short drive away in Los Angeles). Given that length of time to reposition the Wonder to Australia, the fact that those cruises are sold for (relatively) cheap, and that the Australia/New Zealand cruises are generally the lowest priced, I suspect the service will end sooner than later. And the Dream will do Fort Lauderdale, San Diego, Panama Canal and Vancouver.
e.g.
May through mid-September - Alaska from Vancouver
mid-September through October - Mexico from San Diego
first half of November - Panama Canal from San Diego to Fort Lauderdale
second half of November through February - Bahamas/Caribbean from Fort Lauderdale
first half of March - Panama Canal from Fort Lauderdale to San Diego
second half of March through April - Mexico from San Diego