If the city isn't designed for shared transit, even a small trip is going to be awful. SuperShuttle, which picks up 4-5 groups at a time from airports and doesn't use a fixed route, can still add hours to your trip home in LA.
On the other hand, I've used airport busses between transit hubs in cities where everyone takes the subway and it was really quite nice. Better if I ever figure out luggage delivery, of course.
Smaller buses able to optimise routes to make fewer stops will help. Split the current big bus into 3-4 smaller ones and the people going close to the full distance might not stop for the first 80% of stops. That would cut out a lot of time.
Obviously rural areas will be the last to see these changes for non-commercial traffic.