As far as GM "being required to stock parts for vehicles for 15 years", etc. I have posted on this until I am freaking blue in the face, but I'll do it again. HERE ARE FACTS.
1. Yes, I am able to buy GM OEM parts for my 1995 Camaro.
2. In early 2003, I got a letter from Dodge telling me, "We have just concluded building the second generation of Dodge Viper. As we move into building the next generation Viper, we want you to be aware that parts for your Viper are no longer being produced." By the summer of 2003, there were ALREADY certain parts you could not get for the 1992-2002 Vipers. Yes, even though some of those cars were still in warranty. In earlier posts on this subject, I went to www.viperclub.org and found posts dated summer of 2003 PROVING that parts were already unavailable because owners were posting about it. Parts counter employees and people from Dodge posted about it. There is even a thread where people post the specific parts they have found unavailable in that thread, and the contents of that thread were delivered to Dodge to see if they could help. So far, no help.
3. Goto the Corvette people, and ask them about the 1990-1995 ZR1. Parts for the LT5 engine were out of stock within 2 years of the last car coming off the line. Mercury Marine destroyed the dies after making what GM thought was enough spare parts (as they were required to do by GM). Problem was, when word got out that 1995 was the last year for the ZR1, sales jumped, and a lot of the spare parts were used instead to build more complete engines. Thus, certain parts were gone within 2 years.
4. Right now, AT THIS MOMENT, go to your local Saturn Dealer and order a fender for a Sky Redline. There are none. There have been none available for 3 months now. When the plug was pulled on the production of the Sky, GM had to guess how many of each part should be built and stocked for replacement parts. GM had no way to know that so many people would damage the fender on the Sky when they jacked the car up for oil changes. So, GM went thru the fenders at an insanely fast rate. No more can be made because the original supplier is gone.
On the Saturn Sky forum, we have figured out that when a Saturn dealer closes, GM tells the owners to go to their closest Saturn dealer still open. If there is not one within a certain distance, then GM authorizes other GM dealers in that area to do warranty work. So, SOME GM dealers can do Saturn warranty work, and others can't. Depends on how far you are from a Saturn dealer.