Don Brunell

While the merger of United Airlines and Continental left a gaping hole at the ticket counter at SeaTac International Airport, frequent fliers are noticing an expansion of Delta and Alaska check-in stations these days.

That expansion could mean a great boost to Boeing and Washington’s economy in the future — if state lawmakers hold the line on taxes and regulatory costs. According to airport statistics, those two airlines now account for half the passengers flying through Seattle, and they are expanding.

Alaska and Delta have an interesting relationship. Both are using Seattle as hubs and have...