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 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.
When I lived in the San Francisco Bay Area (in San Ramon, in the East Bay), I and many people I knew regularly rode BART to SFO . . . a good 90-minute ride from Dublin/ Pleasanton or Walnut Creek stations. A car trip might be faster on average, but a car trip is also subject to uncertainty . . . i.e., unexpected traffic backups due to accidents or whatever.