yes, there will be a market for parts. the only problem you would have is that when a brand is discontinued, the manufacturers no longer have an investment in that brand, so they may price the parts higher. Basically they no longer need to worry about the brand having a reputation as being expensive to repair. But there will still be competition in terms of the parts suppliers and manufacturers, so I don't think it will be too bad.
The main thing keeping policy makers from going with the restructuring under chapter 11 bankrupcy as the solution to the auto manufacturing crisis is that we are not sure exactly what effect the bankrupcy would have on parts and service, both how it will work for the people who currently own cars of that make, and also what it will do to the decision to buy from a company that has been through bankrupcy. I would expect some negative effect in terms of price and availability of parts, but not a total lack of parts being available at all.