by 1971 the Silverton Branch was isolated from the national rail network as the San Juan Expansion had been abandoned and was being torn out.
There is no way Amtrak would have taken over the operations on the line for that and many other obvious reasons.
The D&RGW would have continued to operate the line.
Service on the line was not really "passenger" service, it was and is a tourist line.
Now a different question:
What if the Silverton line had not been sold in 1980?
I imagine when the D&RGW took over Southern Pacific in 1988 they would have continued to operate the line.
What about when Union Pacific took over SP in 1996? UP was operating their steam fleet out of Cheyenne, would there steam program have been expanded to include the Durango/Siverton operation, or would they have abandoned the line?