The John Day extension might well have extended the life of SVRy, even as a narrow gauge. Mills in John Day and Prairie City are still shipping lumber by truck over the same route. As has been pointed out before, the railway was still turning a profit but the trucks that replaced it never paid.
A line from Prairie City to John Day would have been an easy water level grade, similar to the one up the Sumpter Valley, but a line to Burns would have been Dixie Mtn. all over again. The Oregon & Northwestern line from Burns to Seneca was a piece of work itself, involving a tunnel among other things, and it did not get to the hard part. Hines Lumber Co. lines penitrated closer to John Day, using geared locomotives.
It is interesting to speculate on what might have been had David Eccles survived. He was definitely the push behind the SV, and things were never quite the same with his passing.