Though admittedly not a DL "Fan Boy" - I can admit that this is a considerate move forward for Alaska, as far as DL is concerned.
Of the expansions, most are to the closest (and thus most logical) DL hubs.
A lack of P2P flying here speaks to a much more conventional approach, and while conservative - demonstrates where they see the greatest success, and a focus to commit to what they understand will work. This potentially bases a bedrock for future P2P flying, but based directly (or more concretely) on market demand and dynamics.
Fascinatingly, the A220 would be fully capable of all the flying (including the JFK, weekend runs), however I expect that Alaska's remoteness likely favors cargo - and DL is well equipped to handle that as well. For a start, or for future P2P starts, or for future routes - even starting at A220 chunk sizes, DL has the tools to grow.
Interestingly, these expansions were not to LAX.
One would assume, that if DL were purely out to 'send a message' - then we would be seeing further P2P and going after AS/AA on some of their hubs/key routes. Instead, DL is likely guarding against intrusion on their hub/key routes, early - and building marketshare. Not easy to do, with AS (and brand loyalty), but DL is coming to maturity here, in that they too, have a SEA hub - and a lot to offer. AS is advantaged, with a lower cost base (?), and greater local connectivity. All in all, greater competition and availability for the Alaskan, and those wishing to visit/travel. Here's to a great State, and greater ability to access to it.