Back when I was selling this stuff, I was pretty surprised by how many studios buying the latest and greatest were keeping their old stuff- makes perfect sense, if it's still working, and you can't get a lot in resale, why dump it. I think I probably installed over a hundred, I can only think of about 5 that were trading in or selling the old stuff.
The only studio that I can remember specificaly that was figuring on just flat retiring their old system was one that had the original Leaf DCB2. They were picking up a Canon 20D and a Leaf Aptus 22, it was around 5 years ago I think.
Many times the decision is simply because the new workstations won't support the old hardware- SCSI, OS issues and such...
This is the thing that makes me laugh sometimes about when people complain about their new camera being obsolete by the time you get it home. If it still does the job, it's not obsolete. Heck, I still shoot with my Olympus E10 on occasion...