I expect that most parts will be available for the length of time most people will own them. As with other discontinued makes, as time goes on the trim pieces and body panels become scarcer and scarcer but most mechanical parts should still be available.
Of course, if an aftermarket springs up, as we have seen with say British cars, VW, Mustang, Jeep, etc., the parts crunch might not be too bad at all.
GM has pledged to supply parts and service and will no doubt sell off the tooling to companies that might continue to make parts----hard to say right now how it will all shake out, but in the short term, I don't think there will be severe shortages.
5 years from now? That could start to be a problem.