That's what they always did with express bus service, and it worked quite well.  A typical downtown express route had maybe 3 buses spaced about an hour apart during the morning rush and the same during the evening rush.

But buses are much easier to move around and scale to fit demand. If they put in a route that just didn't prove to be a successful one, they could just relocate it at little cost. With rail, they're stuck.

20-30 years from now it'll probably work better, as transit-dependent development builds up (and gentrifies) around rail stations.  And as the churn of home sales puts more downtown workers closer to stations.  But that's a slow process.