It already takes 40 minutes to go between Westlake and SeaTac stations, which I find bearable but not great when using Link for personal trips. When returning at night or for business travel, I'm likely going to take a Lyft or use off-site parking with a discount that gets it down to about $10/day.
I very rarely pay more than around $10/day for off-site parking at any of several lots that are an easy walk from the terminal or the light rail station
that said, Uber/Lyft from Magnolia was close to $80 on Tues afternoon 
I can't imagine the north end and eastside extensions will have many airport riders between how long the ride will be and needing to Lyft/Uber to a station.
I think this will fall into the “It depends” category … those who can ride w friends or family to/from the suburban stations might be more likely to use the light rail than those who need to call a Lyft or an Uber anyway