It's a moot point for me at this point since my 2011 "reflash" went south and dealer is waiting on Sweden to tell them how to fix it. I wanted my RDS but....hmmm.
As far as parts interchangeability goes I imagine a lot of maintenance items, electrical items, relays, sensors, and some hoses (which have GM numbers stamped on them) would be able to be crossed to something by I don't know by whom and it might not be straightforward crosses. For example It could get difficult to cross when the part is the same but the pigtail is unique to the 9-5 so the number is different - then you have to get creative to find it and then make it work. Some parts will be aftermarket like brake pads, maybe brake hydraulics and chassis wear parts.
Body parts, trim parts and instrument cluster will be impossible to cross for obvious reasons.
Unless the parts situation vastly improves with time (unlikely without a restart of production) I'm not so sure it will be a car you can drive 200K miles - not saying it couldn't last that long, my current issue aside it is a solid car but if you cant get a crucial part to fix it then it's stopped in its own tracks regardless. And its too early to tell what the weak points are in order to hoard anything and the likelihood of finding a parts car is pretty slim at the moment. That could change when insurance companies total these cars out for a relatively minor accident due to parts availability but then you have to be in the right place at the right time.