I think it's already very convenient for commuters up and down the Danbury line to drive over to the Harlem line. Some of my recent research on the Beacon line has articles about the hunt for parking up and down the Harlem, and communities resisting expansion of roads and lots.
I remember back in 91 when I lived in Patterson, sometimes I'd drive down to (then) Brewster North and park. If I remember, they were parking along the road at that time. I think they've since expanded the lot and prohibited that (if it wasn't prohibited at that time).
It kind of points out the folly of improving one line and ignoring another. If they would quite studying and upgrade the Danbury Line (including travel times , frequency, and parking), they'd eliminate a lot of folks from CT migrating over to the Harlem.
There's also the matter of destination. If you're in Danbury or along that line, and working in White Plains, the Danbury line doesn't do you much good. Unless you want to finish the Westchester Northern
stillborn in the 1920's, you've got to revisit the Beacon line.