The Pioneer and Desert Wind were cut in (IIRC) 1997 during the period of $-0- budgets and "self-sufficency". In fact the 180 day rule for cutting a route was put into place, because these routes shut down without warning!
I'm pretty sure the 180-day rule predates Amtrak.
There certainly was warning that the Desert Wind and Pioneer would be shut down. In fact, there was a discontinuance notice posted for the Texas Eagle, which would have shut down at the same time, if not for a grassroots effort by residents along the line to convince the Texas to provide Amtrak a loan to keep the route running.
The $0 proposed budgets were during the Bush era, and (I think) the Reagan era.
The Desert Wind and Pioneer were cut, in part, because Amtrak needed the equipment to make other trains (the Empire Builder and California Zephyr to Emeryville) daily. If they hadn't been cut, and no new equipment was acquired in the mean time, then you'd have 4x/week service west of St. Paul, and 3x/week service on a lot of the other lines. There simply wouldn't be enough equipment to do anything else.
Even though there are some connectivity gaps in the system, being able to offer daily service along the rest of the routes has resulted in significantly improved revenue and efficiency.
The 180 day rule was instituted as part of the National Railroad Passenger Act of 1970 establishing Amtrak. Prior to Amtrak, on interstate trains, the railroads had to post a Notice of Discontinuation, which were usually set for 30 days from the date of the posting. Then somebody, sometimes many somebodies would protest the notice and the ICC would stay it until a hearing, usually some months away. Then there was the hearing and there was another wait until the ICC ruling from the hearing, and frequently the RRs were turned down, had to keep running the train, and start over again. Sometimes the ICC would direct that the railroads perform some action and reapply. Sometimes the notices were not contested, such as Santa Fe's Fast Mail, in which case the train just came off on the stated date. Once ICC approval was given, the trains were usually gone in a week or two after the ruling. I think the Chief came off about 1 week after the ruling.
Amtrak would have been subject to the same regulation and protests and hearings had Congress not changed it for Amtrak. All Amtrak had to do was post the 180 notice and wait it out. Congress wanted to give Amtrak the flexibility to control its routes (under Congressional supervision, of course, see The Hilltopper). The remaining RR intercity trains on the Southern, the D&RGW, and the Rock Island were not included and their train-offs had to go through the old procedure. Also the Act also made law that directed the ICC to not consider ANY train-offs from non-participating carriers until 1975. If they didn't joing Amtrak, they'd be stuck with whatever they were running. That was the stick. In point of fact, the ICC did not grant many train off petitions until an RR joined Amtrak (I think Southern was able to get of some they started with in 1971, but not all of them), although they did not have jurisdiction over the Rock Island, because their trains ran within Illinois only and they were able to get rid of them in the late 70s. The D&RGW tried many times to get rid of the RGZ entirely or cut it back to Grand Junction before deciding to join Amtrak, effective April 1983. Then the ICC immediately approved the discontinuation in conjunction with the start of Amtrak service (which didn't actually start over the route until some months later because of the Thistle slide severing the line in Utah).
Had the railroads had the same 180-rule that Amtrak has, there would have been virtually no intercity passenger trains at all by April 1968 at the latest. That being 6 months after the infamous October 1967 action of the Post Office removing most First Class mail from the trains, which kicked the last financial legs out from under the passenger train. They all would have headed for the exits then, if they hadn't earlier.
As to the Desert Wind and Pioneer, the reasons were the "Glidepath to Self Suffiency", the damage done by the infamous Mercer cuts when most western LD trains, at least, were reduced to less than daily operation (the Builder ran 4 days a week, the Pioneer ran 3 days a week, for example. On the days the Pioneer/Desert Wind ran, there was no California Zephyr, no train west of Salt Lake to Oakland). That played havoc with ridership because no one knew when the trains ran. Finally, Amtrak retired most of the Hi-Level fleet inherited from Santa Fe that had been padding out the Superliner fleet when they made the Mercer cuts. When it became obvious the Mercer cuts were a horrendous mistake and the trains should run daily (except those like the Sunset and Cardinal that had been less than daily before the cuts), they no longer had the equipment to run all trains daily that had run before the Mercer cuts because the Hi-Levels were mostly gone.
By the way, the "Carter Cuts" in 1979, such as the National Limited, the Floridian, the North Coast Hiawatha were done using the 180-day notice. It was in the law and available to Amtrak from day 1.