If they (D&RGW) had applied for a straight discontinuance, it's highly likely that the ICC would have forced them to continue operating the train...IIRC, they did formally apply to end service between Grand Junction and Salt Lake City and were turned down. Hence, they came to a meeting of the minds with Amtrak.
The main factor in the ultimate "throwing in of the towel" was, IIRC, that their diner broke down and repairs were going to be uneconomical. They did apply to end service past Grand Junction and got refused (I think this was in the context of the petition fights over the California Zephyr
, which repeatedly failed due to overall high seasonal ridership until the petition was broken up ), and I think the pax numbers on the eastern end of the route were such that a discontinuation petition would have been doomed. The other thing that was a major factor in 1971 was that I believe Amtrak was seriously looking at a second train on the route (remember, the Denver Zephyr
ran separately on a seasonal basis for a year or two, Amtrak experimented with a revived Chief
opposite the Super Chief
in the mid-1970s as well, and Amtrak seriously
looked at running a Kansas City-Denver train on top of that, so the threat wasn't hollow ). This was rather less of a risk in 1983.
 When the petition got granted, WP got out entirely. DRGW got to go to 3x/week (they tried to dump everything west of Grand Junction but were required to keep, I believe the phrase was, "a semblance of service". CBQ kept daily service. Basically, the overall petition was screwed because there was way
too much ridership east of Grand Junction to let the train go, even if WP was stuck with a massive hole on the western end.
 It isn't implausible to imagine such a service being seasonal, or to see it getting run KCY-DEN-SLC-SEA or somesuch. Don't forget how relatively fluid the network was in its first decade or so.