If B6/VX happened then Alaska would have been pigeonholed in SEA/PDX with DL growing larger and nowhere to reasonably expand with a new west coast competitor likely to enter/grow many short haul west coast markets further fragmenting the market.

AS buying VX blocked B6, who has completely failed to form a west coast short haul strategy since then (LGB is a disaster, and likely to disappear completely). B6 has grown their transcon market share, which has been lucrative and something AS hasn't really seemed to focus on.

In hindsight, I think AS bought VX out of fear of the possible outcome vs the probable outcome. B6 wanted out from LGB, and if they merged with VX then I think the transcon flying would have expanded more than we have seen already along with Hawaii flying and leisure Mexico type destinations from LAX/SFO leaving LGB as a ghost town. Short haul west coast was never going to be a big segment for the combined carrier, but AS was worried mostly about that.