no one said anything about domination.
somehow I doubt that DL became interested in LAX-YVR just because they will have some E175s sitting around for Christmas.
maybe DL will do more with the route than just the winter holidays; maybe not. Given that AA just announced LAX-YVR, DL's timing is, um, interesting.
there is a competitive reaction to many actions that are made in the industry, including AA's decision to put one of its new 772ERs on the DFW-SCL route, coincidentally as UA announces new/restarted service to SCL.